linux-kernel - Re: [PATCH 3/4] KVM: x86: drop bogus MWAIT check

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Date:   Thu, 4 May 2017 16:33:28 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Alexander Graf <agraf@...e.de>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        "Gabriel L. Somlo" <gsomlo@...il.com>
Subject: Re: [PATCH 3/4] KVM: x86: drop bogus MWAIT check

2017-05-04 12:58+0200, Paolo Bonzini:
> On 03/05/2017 21:37, Radim Krčmář wrote:
>> The guest can call MWAIT with ECX = 0 even if we enforce
>> CPUID5_ECX_INTERRUPT_BREAK;  the call would have the exactly the same
>> effect as if the host didn't have CPUID5_ECX_INTERRUPT_BREAK.
>> 
>> The check was added in some iteration while trying to fix a reported
>> OS X on Core 2 bug, but the CPU had CPUID5_ECX_INTERRUPT_BREAK and the
>> bug is elsewhere.
> 
> The reason for this, as I understood it, is that we have historically
> not published leaf 5 information via KVM_GET_SUPPORTED_CPUID.  For this
> reason, QEMU is publishing CPUID5_ECX_INTERRUPT_BREAK.  Then if:

I see, it was added to QEMU in e737b32a3688 ("Core 2 Duo specification
(Alexander Graf)").

> - the host doesn't have ECX[0]=1 support
> 
> - the guest sets ECX[0]
> 
> you get a #GP in the guest.  So wrong comment but right thing to do.

That userspace didn't set CPUID.01H:ECX.MONITOR[bit 3], so a guest
should get #UD instead, but MWAIT couldn't be expected to work.

I think that the guest bug is very unlikely, so I'd get rid of the
condition anyway ... we have also recently killed support for pre-Core 2
hosts and AFAIK, all newer Intels have it.

(Not so sure about AMDs, which share the same problem, so we do need to
 do more than just comment it better in any case.)