linux-kernel - Re: [PATCH] KVM: x86: work around QEMU issue with synthetic CPUID leaves

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Message-ID: <54adc4a3-6b66-8ddf-db92-9630089da2dd@redhat.com>
Date:   Sun, 1 May 2022 19:37:02 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: work around QEMU issue with synthetic CPUID
 leaves

On 5/1/22 13:16, Maxim Levitsky wrote:
>> +		 * However, only do it if the host has CPUID leaf 0x8000001d.
>> +		 * QEMU thinks that it can query the host blindly for that
>> +		 * CPUID leaf if KVM reports that it supports 0x8000001d or
>> +		 * above.  The processor merrily returns values from the
>> +		 * highest Intel leaf which QEMU tries to use as the guest's
>> +		 * 0x8000001d.  Even worse, this can result in an infinite
>> +		 * loop if said highest leaf has no subleaves indexed by ECX.
>
> Very small nitpick: It might be useful to add a note that qemu does this only for the
> leaf 0x8000001d.

Yes, it's there: "QEMU thinks that it can query the host blindly for 
that CPUID leaf", "that" is 0x8000001d in the previous sentence.

Paolo