Re: [RFC][PATCH] kvm: x86: Improve emulation of CPUID leaves 0BH and 1FH

From: Jim Mattson <jmattson@google.com>
To: kvm list <kvm@vger.kernel.org>
Cc: Steve Rutherford <srutherford@google.com>,
	Jacob Xu <jacobhxu@google.com>, Peter Shier <pshier@google.com>
Subject: Re: [RFC][PATCH] kvm: x86: Improve emulation of CPUID leaves 0BH and 1FH
Date: Wed, 18 Sep 2019 08:32:58 -0700	[thread overview]
Message-ID: <CALMp9eRz02uUbWdF_tfyoj0y1bfgeg3swsHW1wqxkSJQk-PrfQ@mail.gmail.com> (raw)
In-Reply-To: <20190912232753.85969-1-jmattson@google.com>

On Thu, Sep 12, 2019 at 4:28 PM Jim Mattson <jmattson@google.com> wrote:
>
> If these CPUID leaves are implemented, the EDX output is always the
> x2APIC ID, regardless of the ECX input. Furthermore, the low byte of
> the ECX output is always identical to the low byte of the ECX input.
>
> KVM's CPUID emulation doesn't report the correct ECX and EDX outputs
> when the ECX input is greater than the first subleaf for which the
> "level type" is zero. This is probably only significant in the case of
> the x2APIC ID, which should be the result of CPUID(EAX=0BH):EDX or
> CPUID(EAX=1FH):EDX, without even setting a particular ECX input value.
>
> Create a "wildcard" kvm_cpuid_entry2 for leaves 0BH and 1FH in
> response to the KVM_GET_SUPPORTED_CPUID ioctl. This entry does not
> have the KVM_CPUID_FLAG_SIGNIFCANT_INDEX flag, so it matches all
> subleaves for which there isn't a prior explicit index match.
>
> Add a new KVM_CPUID flag that is only applicable to leaves 0BH and
> 1FH: KVM_CPUID_FLAG_CL_IS_PASSTHROUGH. When KVM's CPUID emulation
> encounters this flag, it will fix up ECX[7:0] in the CPUID output. Add
> this flag to the aforementioned "wildcard" kvm_cpuid_entry2.
>
> Note that userspace is still responsible for setting EDX to the x2APIC
> ID of the vCPU in each of these structures, *including* the wildcard.
>
> Qemu doesn't pass the flags from KVM_GET_SUPPORTED_CPUID to
> KVM_SET_CPUID2, so it will have to be modified to take advantage of
> these changes. Note that passing the new flag to older kernels will
> have no effect.
>
> Unfortunately, the new flag bit was not previously reserved, so it is
> possible that a userspace agent that already sets this bit will be
> unhappy with the new behavior. Technically, I suppose, this should be
> implemented as a new set of ioctls. Posting as an RFC to get comments
> on the API breakage.
>
> Fixes: 0771671749b59a ("KVM: Enhance guest cpuid management")
> Fixes: a87f2d3a6eadab ("KVM: x86: Add Intel CPUID.1F cpuid emulation support")
> Signed-off-by: Jim Mattson <jmattson@google.com>
> Reviewed-by: Steve Rutherford <srutherford@google.com>
> Reviewed-by: Jacob Xu <jacobhxu@google.com>
> Reviewed-by: Peter Shier <pshier@google.com>

No comments on the API breakage? Shall I resubmit as an actual patch?