[1/2] KVM: SVM: Fix TSC_AUX virtualization setup

  The checks for virtualizing TSC_AUX occur during the vCPU reset processing
path. However, at the time of initial vCPU reset processing, when the vCPU
is first created, not all of the guest CPUID information has been set. In
this case the RDTSCP and RDPID feature support for the guest is not in
place and so TSC_AUX virtualization is not established.

This continues for each vCPU created for the guest. On the first boot of
an AP, vCPU reset processing is executed as a result of an APIC INIT
event, this time with all of the guest CPUID information set, resulting
in TSC_AUX virtualization being enabled, but only for the APs. The BSP
always sees a TSC_AUX value of 0 which probably went unnoticed because,
at least for Linux, the BSP TSC_AUX value is 0.

Move the TSC_AUX virtualization enablement into the vcpu_after_set_cpuid()
path to allow for proper initialization of the support after the guest
CPUID information has been set.

Fixes: 296d5a17e793 ("KVM: SEV-ES: Use V_TSC_AUX if available instead of RDTSC/MSR_TSC_AUX intercepts")
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
---
 arch/x86/kvm/svm/sev.c | 27 +++++++++++++++++++--------
 arch/x86/kvm/svm/svm.c |  3 +++
 arch/x86/kvm/svm/svm.h |  1 +
 3 files changed, 23 insertions(+), 8 deletions(-)
  

On 9/14/23 16:13, Sean Christopherson wrote:
> On Thu, Sep 14, 2023, Tom Lendacky wrote:
>> On 9/14/23 15:28, Sean Christopherson wrote:
>>> On Thu, Sep 14, 2023, Tom Lendacky wrote:
>>>> The checks for virtualizing TSC_AUX occur during the vCPU reset processing
>>>> path. However, at the time of initial vCPU reset processing, when the vCPU
>>>> is first created, not all of the guest CPUID information has been set. In
>>>> this case the RDTSCP and RDPID feature support for the guest is not in
>>>> place and so TSC_AUX virtualization is not established.
>>>>
>>>> This continues for each vCPU created for the guest. On the first boot of
>>>> an AP, vCPU reset processing is executed as a result of an APIC INIT
>>>> event, this time with all of the guest CPUID information set, resulting
>>>> in TSC_AUX virtualization being enabled, but only for the APs. The BSP
>>>> always sees a TSC_AUX value of 0 which probably went unnoticed because,
>>>> at least for Linux, the BSP TSC_AUX value is 0.
>>>>
>>>> Move the TSC_AUX virtualization enablement into the vcpu_after_set_cpuid()
>>>> path to allow for proper initialization of the support after the guest
>>>> CPUID information has been set.
>>>>
>>>> Fixes: 296d5a17e793 ("KVM: SEV-ES: Use V_TSC_AUX if available instead of RDTSC/MSR_TSC_AUX intercepts")
>>>> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
>>>> ---
>>>>    arch/x86/kvm/svm/sev.c | 27 +++++++++++++++++++--------
>>>>    arch/x86/kvm/svm/svm.c |  3 +++
>>>>    arch/x86/kvm/svm/svm.h |  1 +
>>>>    3 files changed, 23 insertions(+), 8 deletions(-)
>>>>
>>>> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
>>>> index b9a0a939d59f..565c9de87c6d 100644
>>>> --- a/arch/x86/kvm/svm/sev.c
>>>> +++ b/arch/x86/kvm/svm/sev.c
>>>> @@ -2962,6 +2962,25 @@ int sev_es_string_io(struct vcpu_svm *svm, int size, unsigned int port, int in)
>>>>    				    count, in);
>>>>    }
>>>> +static void sev_es_init_vmcb_after_set_cpuid(struct vcpu_svm *svm)
>>>
>>> I would rather name this sev_es_after_set_cpuid() and call it directly from
>>> svm_vcpu_after_set_cpuid().  Or I suppose bounce through sev_after_set_cpuid(),
>>> but that seems gratuitous.
>>
>> There is a sev_guest() check in svm_vcpu_after_set_cpuid(), so I can move
>> that into sev_vcpu_after_set_cpuid() and keep the separate
>> sev_es_vcpu_after_set_cpuid().
> 
> Works for me.
> 
>> And it looks like you would prefer to not have "vcpu" in the function name?
>> Might be better search-wise if vcpu remains part of the name?
> 
> Oh, that was just a typo/oversight, not intentional.
> 
>>> AFAICT, there's no point in calling this from init_vmcb(); guest_cpuid_has() is
>>> guaranteed to be false when called during vCPU creation and so the intercept
>>> behavior will be correct, and even if SEV-ES called init_vmcb() from
>>> shutdown_interception(), which it doesn't, guest_cpuid_has() wouldn't change,
>>> i.e. the intercepts wouldn't need to be changed.
>>
>> Ok, I thought that's how it worked, but wasn't 100% sure. I'll move it out
>> of the init_vmcb() path.
>>
>>>
>>> init_vmcb_after_set_cpuid() is a special snowflake because it handles both SVM's
>>> true defaults *and* guest CPUID updates.
>>>
>>>> +{
>>>> +	struct kvm_vcpu *vcpu = &svm->vcpu;
>>>> +
>>>> +	if (boot_cpu_has(X86_FEATURE_V_TSC_AUX) &&
>>>> +	    (guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP) ||
>>>> +	     guest_cpuid_has(vcpu, X86_FEATURE_RDPID))) {
>>>> +		set_msr_interception(vcpu, svm->msrpm, MSR_TSC_AUX, 1, 1);
>>>
>>> This needs to toggled interception back on if RDTSCP and RDPID are hidden from
>>> the guest.  KVM's wonderful ABI doesn't disallow multiple calls to KVM_SET_CPUID2
>>> before KVM_RUN.
>>
>> Do you want that as a separate patch with the first patch purely addressing
>> the current issue? Or combine them?
> 
> Hmm, now that you mention it, probably a seperate patch on top.

This toggling possibility raises a question related to the second patch in 
this series that eliminates the use of the user return MSR for TSC_AUX. 
Depending on when the interfaces are called (set CPUID, host-initiated 
WRMSR of TSC_AUX, set CPUID again), I think we could end up in a state 
where the host TSC_AUX may not get restored properly, not 100% sure at the 
moment, though.

Let me drop that patch from the series for now and just send the fix(es). 
I'll work through the other scenarios and code paths and send the user 
return MSR optimization as a separate series later.

Thanks,
Tom
  

On Fri, Sep 15, 2023, Tom Lendacky wrote:
> On 9/14/23 16:13, Sean Christopherson wrote:
> This toggling possibility raises a question related to the second patch in
> this series that eliminates the use of the user return MSR for TSC_AUX.
> Depending on when the interfaces are called (set CPUID, host-initiated WRMSR
> of TSC_AUX, set CPUID again), I think we could end up in a state where the
> host TSC_AUX may not get restored properly, not 100% sure at the moment,
> though.

Give me a few minutes to respond to patch 2, I think it can be much simpler, more
performant, and avoid any races.

> Let me drop that patch from the series for now and just send the fix(es).
> I'll work through the other scenarios and code paths and send the user
> return MSR optimization as a separate series later.
  

On 9/14/23 15:48, Tom Lendacky wrote:
> On 9/14/23 15:28, Sean Christopherson wrote:
>> On Thu, Sep 14, 2023, Tom Lendacky wrote:

> 
>>
>>> +        if (guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
>>> +            svm_clr_intercept(svm, INTERCEPT_RDTSCP);
>>
>> Same thing here.
> 
> Will do.

For RDTSCP, svm_recalc_instruction_intercepts() will set/clear the RDTSCP 
intercept as part of the svm_vcpu_set_after_cpuid() path, but it will only 
do it based on kvm_cpu_cap_has(X86_FEATURE_RDTSCP) being true, which is 
very likely.

Do you think that is good enough and we can drop the setting and clearing 
of the RDTSCP intercept in the sev_es_vcpu_set_after_cpuid() function and 
only deal with the TSC_AUX MSR intercept?

On a side note, it looks like RDTSCP would not be intercepted if the KVM 
cap X86_FEATURE_RDTSCP feature is cleared, however unlikely, in 
kvm_set_cpu_caps() and RDTSCP is not advertised to the guest (assuming the 
guest is ignoring the RDTSCP CPUID bit).

Thanks,
Tom

> 
> Thanks,
> Tom
> 
>>
>>> +    }
>>> +}
  

On Fri, Sep 15, 2023, Tom Lendacky wrote:
> On 9/14/23 15:48, Tom Lendacky wrote:
> > On 9/14/23 15:28, Sean Christopherson wrote:
> > > On Thu, Sep 14, 2023, Tom Lendacky wrote:
> 
> > 
> > > 
> > > > +        if (guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
> > > > +            svm_clr_intercept(svm, INTERCEPT_RDTSCP);
> > > 
> > > Same thing here.
> > 
> > Will do.
> 
> For RDTSCP, svm_recalc_instruction_intercepts() will set/clear the RDTSCP
> intercept as part of the svm_vcpu_set_after_cpuid() path, but it will only
> do it based on kvm_cpu_cap_has(X86_FEATURE_RDTSCP) being true, which is very
> likely.
> 
> Do you think that is good enough and we can drop the setting and clearing of
> the RDTSCP intercept in the sev_es_vcpu_set_after_cpuid() function and only
> deal with the TSC_AUX MSR intercept?

The common handling should be good enough.

> On a side note, it looks like RDTSCP would not be intercepted if the KVM cap
> X86_FEATURE_RDTSCP feature is cleared, however unlikely, in
> kvm_set_cpu_caps() and RDTSCP is not advertised to the guest (assuming the
> guest is ignoring the RDTSCP CPUID bit).

Hmm, yes, though the only scenario in which KVM clears RDTSCP on AMD comes with
a WARN (it's a guard against KVM bugs).  If the guest ignores CPUID and uses
RDTSCP anyways, the guest deserves its death, and leaking the host pCPU doesn't
seem like a major issue.

That said, if hardware behavior is to ignore unknown intercepts, e.g. if KVM can
safely set INTERCEPT_RDTSCP even when hardware doesn't support said intercept,
then I wouldn't be opposed to doing:

	/*
	 * Intercept INVPCID if shadow paging is enabled to sync/free shadow
	 * roots, or if INVPCID is disabled in the guest to inject #UD.
	 */
	if (!kvm_cpu_cap_has(X86_FEATURE_INVPCID) ||
	    !npt_enabled || !guest_cpuid_has(&svm->vcpu, X86_FEATURE_INVPCID))
		svm_set_intercept(svm, INTERCEPT_INVPCID);
	else
		svm_clr_intercept(svm, INTERCEPT_INVPCID);

	if (kvm_cpu_cap_has(X86_FEATURE_RDTSCP) &&
	    guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
		svm_clr_intercept(svm, INTERCEPT_RDTSCP);
	else
		svm_set_intercept(svm, INTERCEPT_RDTSCP);

Alternatively, KVM could check boot_cpu_has() instead or kvm_cpu_cap_has(), but
that's not foolproof either, e.g. see Intel's of hiding PCID to workaround the
TLB flushing bug on Alderlake.  So my vote would either be to keep things as-is,
or do the above (if that's safe).
  

On 9/15/23 12:32, Sean Christopherson wrote:
> On Fri, Sep 15, 2023, Tom Lendacky wrote:
>> On 9/14/23 15:48, Tom Lendacky wrote:
>>> On 9/14/23 15:28, Sean Christopherson wrote:
>>>> On Thu, Sep 14, 2023, Tom Lendacky wrote:
>>
>>>
>>>>
>>>>> +        if (guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
>>>>> +            svm_clr_intercept(svm, INTERCEPT_RDTSCP);
>>>>
>>>> Same thing here.
>>>
>>> Will do.
>>
>> For RDTSCP, svm_recalc_instruction_intercepts() will set/clear the RDTSCP
>> intercept as part of the svm_vcpu_set_after_cpuid() path, but it will only
>> do it based on kvm_cpu_cap_has(X86_FEATURE_RDTSCP) being true, which is very
>> likely.
>>
>> Do you think that is good enough and we can drop the setting and clearing of
>> the RDTSCP intercept in the sev_es_vcpu_set_after_cpuid() function and only
>> deal with the TSC_AUX MSR intercept?
> 
> The common handling should be good enough.
> 
>> On a side note, it looks like RDTSCP would not be intercepted if the KVM cap
>> X86_FEATURE_RDTSCP feature is cleared, however unlikely, in
>> kvm_set_cpu_caps() and RDTSCP is not advertised to the guest (assuming the
>> guest is ignoring the RDTSCP CPUID bit).
> 
> Hmm, yes, though the only scenario in which KVM clears RDTSCP on AMD comes with
> a WARN (it's a guard against KVM bugs).  If the guest ignores CPUID and uses
> RDTSCP anyways, the guest deserves its death, and leaking the host pCPU doesn't
> seem like a major issue.
> 
> That said, if hardware behavior is to ignore unknown intercepts, e.g. if KVM can
> safely set INTERCEPT_RDTSCP even when hardware doesn't support said intercept,
> then I wouldn't be opposed to doing:
> 
> 	/*
> 	 * Intercept INVPCID if shadow paging is enabled to sync/free shadow
> 	 * roots, or if INVPCID is disabled in the guest to inject #UD.
> 	 */
> 	if (!kvm_cpu_cap_has(X86_FEATURE_INVPCID) ||
> 	    !npt_enabled || !guest_cpuid_has(&svm->vcpu, X86_FEATURE_INVPCID))
> 		svm_set_intercept(svm, INTERCEPT_INVPCID);
> 	else
> 		svm_clr_intercept(svm, INTERCEPT_INVPCID);
> 
> 	if (kvm_cpu_cap_has(X86_FEATURE_RDTSCP) &&
> 	    guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
> 		svm_clr_intercept(svm, INTERCEPT_RDTSCP);
> 	else
> 		svm_set_intercept(svm, INTERCEPT_RDTSCP);
> 
> Alternatively, KVM could check boot_cpu_has() instead or kvm_cpu_cap_has(), but
> that's not foolproof either, e.g. see Intel's of hiding PCID to workaround the
> TLB flushing bug on Alderlake.  So my vote would either be to keep things as-is,
> or do the above (if that's safe).

Keep things as-is works for me :)

Thanks,
Tom