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Date:   Wed, 13 Dec 2023 15:10:25 -0800
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Subject: Re: [PATCH v2 1/3] KVM: x86: Make the hardcoded APIC bus frequency vm variable
From:   Sean Christopherson <seanjc@google.com>
To:     Maxim Levitsky <mlevitsk@redhat.com>
Cc:     isaku.yamahata@intel.com, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, isaku.yamahata@gmail.com,
        Paolo Bonzini <pbonzini@redhat.com>, erdemaktas@google.com,
        Vishal Annapurve <vannapurve@google.com>,
        Jim Mattson <jmattson@google.com>
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On Thu, Dec 14, 2023, Maxim Levitsky wrote:
> On Mon, 2023-11-13 at 20:35 -0800, isaku.yamahata@intel.com wrote:
> > From: Isaku Yamahata <isaku.yamahata@intel.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.

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 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