Re: [PATCH v6 1/1] x86: kvm: svm: set up ERAPS support for guests

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 21/11/2025 2:58 pm, Sean Christopherson wrote:
> On Fri, Nov 21, 2025, Andrew Cooper wrote:
>> On 20/11/2025 8:11 pm, Sean Christopherson wrote:
>>> The emulation requirements are not limited to shadow paging.  From the APM:
>>>
>>>   The ERAPS feature eliminates the need to execute CALL instructions to clear
>>>   the return address predictor in most cases. On processors that support ERAPS,
>>>   return addresses from CALL instructions executed in host mode are not used in
>>>   guest mode, and vice versa. Additionally, the return address predictor is
>>>   cleared in all cases when the TLB is implicitly invalidated (see Section 5.5.3 “TLB
>>>   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>>   Management,” on page 159) and in the following cases:
>>>
>>>   • MOV CR3 instruction
>>>   • INVPCID other than single address invalidation (operation type 0)
>> I already asked AMD for clarification here.  AIUI, INVLPGB should be
>> included in this list, and that begs the question what else is missed
>> from the documentation.
>>
>>> Yes, KVM only intercepts MOV CR3 and INVPCID when NPT is disabled (or INVPCID is
>>> unsupported per guest CPUID), but that is an implementation detail, the instructions
>>> are still reachable via emulator, and KVM needs to emulate implicit TLB flush
>>> behavior.
>> The Implicit flushes cover CR0.PG, CR4.{PSE,PGE,PCIDE,PKE}, SMI, RSM,
>> writes to MTRR MSR, #INIT, A20M, and "other model specific MSRs, see NDA
>> docs".
>>
>> The final part is very unhelpful in practice, and necessitates a RAS
>> flush on any emulated WRMSR, unless AMD are going to start handing out
>> the multi-coloured documents...
> Does Xen actually emulate guest TLB flushes on all emulated WRMSRs?

Not currently.  I need to reassess in light of this conversation.

> A RAS flush seems like small peanuts compared to a TLB flush.

Workload dependent, but in the common case, I'd expect so.

>
>> The really fastpath MSRs are unintercepted and won't suffer this overhead.
> Heh, if an unintercepted MSR is on the "naughty list", wouldn't that break shadow
> paging schemes that rely on intercepting architectural TLB flushes to synchronize
> shadow PTEs?

Hmm.  Yes it would, if (and only if) the OS is aware of and depending on
the WRMSR for TLB flushing.

I doubt OSes are depending on model specific side effects such as this,
but we have no way to know for sure.

~Andrew