linux-kernel - Re: [PATCH v2 1/3] KVM: x86: Make the hardcoded APIC bus frequency vm variable

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Date:   Thu, 14 Dec 2023 08:41:43 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     isaku.yamahata@...el.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, isaku.yamahata@...il.com,
        Paolo Bonzini <pbonzini@...hat.com>, erdemaktas@...gle.com,
        Vishal Annapurve <vannapurve@...gle.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH v2 1/3] KVM: x86: Make the hardcoded APIC bus frequency vm variable

On Thu, Dec 14, 2023, Maxim Levitsky wrote:
> On Wed, 2023-12-13 at 15:10 -0800, Sean Christopherson wrote:
> > Upstream KVM's non-TDX behavior is fine, because KVM doesn't advertise support
> > for CPUID 0x15, i.e. doesn't announce to host userspace that it's safe to expose
> > CPUID 0x15 to the guest.  Because TDX makes exposing CPUID 0x15 mandatory, KVM
> > needs to be taught to correctly emulate the guest's APIC bus frequency, a.k.a.
> > the TDX guest core crystal frequency of 25Mhz.
> 
> I assume that TDX doesn't allow to change the CPUID 0x15 leaf.

Correct.  I meant to call that out below, but left my sentence half-finished.  It
was supposed to say:

  I halfheartedly floated the idea of "fixing" the TDX module/architecture to either
  use 1Ghz as the base frequency or to allow configuring the base frequency
  advertised to the guest.

> > I halfheartedly floated the idea of "fixing" the TDX module/architecture to either
> > use 1Ghz as the base frequency (off list), but it definitely isn't a hill worth
> > dying on since the KVM changes are relatively simple.
> > 
> > https://lore.kernel.org/all/ZSnIKQ4bUavAtBz6@google.com
> > 
> 
> Best regards,
> 	Maxim Levitsky
>