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 11:31:24 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.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 Wed, 2023-12-13 at 15:10 -0800, Sean Christopherson wrote:
> On Thu, Dec 14, 2023, Maxim Levitsky wrote:
> > On Mon, 2023-11-13 at 20:35 -0800, isaku.yamahata@...el.com wrote:
> > > From: Isaku Yamahata <isaku.yamahata@...el.com>
> > > 
> > > TDX virtualizes the advertised APIC bus frequency to be 25MHz. 
> > 
> > Can you explain a bit better why TDX needs this? I am not familiar
> > with TDX well enough yet to fully understand.
> 
> TDX (the module/architecture) hardcodes the core crystal frequency to 25Mhz,
> whereas KVM hardcodes the APIC bus frequency to 1Ghz.  And TDX (again, the module)
> *unconditionally* enumerates CPUID 0x15 to TDX guests, i.e. _tells_ the guest that
> the frequency is 25MHz regardless of what the VMM/hypervisor actually emulates.
> And so the guest skips calibrating the APIC timer, which results in the guest
> scheduling timer interrupts waaaaaaay too frequently, i.e. the guest ends up
> gettings interrupts at 40x the rate it wants.

That is what I wanted to hear without opening the PRM ;) - so there is a CPUID leaf,
but KVM just doesn't advertise it. Now it makes sense.

Please add something like that to the commit message:

"TDX guests have the APIC bus frequency hardcoded to 25 Mhz in the CPUID leaf 0x15.
KVM doesn't expose this leaf, but TDX mandates it to be exposed,
and doesn't allow to override it's value either.

To ensure that the guest doesn't have a conflicting view of the APIC bus frequency, 
allow the userspace to tell KVM to use the same frequency that TDX mandates,
instead of the default 1Ghz"

> 
> 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.

> 
> 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