linux-kernel - Re: [PATCH 0/2] KVM: x86: emulate fxsave and fxrstor

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Message-ID: <20161027164059.GG4212@potion>
Date:   Thu, 27 Oct 2016 18:41:00 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Laszlo Ersek <lersek@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 0/2] KVM: x86: emulate fxsave and fxrstor

2016-10-26 23:40+0200, Laszlo Ersek:
> On 10/26/16 22:50, Radim Krčmář wrote:
>> [1/2] adds the emulation (and could be split into two patches if you'd like),
>> [2/2] just refactors the code.
>> 
>> This should fix an issue that users are hitting.  Laszlo found several reports:
>>  - https://bugs.launchpad.net/qemu/+bug/1623276
>>  - https://bugzilla.proxmox.com/show_bug.cgi?id=1182
>>  - https://bugs.archlinux.org/task/50778
>> 
>> I have only tested it with a simple kvm-unit-tests, though.  Reproducing the
>> iPXE issue is on the way ...
>> 
>> 
>> Radim Krčmář (2):
>>   KVM: x86: emulate fxsave and fxrstor
>>   KVM: x86: save one bit in ctxt->d
>> 
>>  arch/x86/kvm/emulate.c | 110 ++++++++++++++++++++++++++++++++++++++++++-------
>>  1 file changed, 94 insertions(+), 16 deletions(-)
>> 
> 
> I was just about to post iPXE patches that would disable the FXSAVE /
> FXRSTOR instructions in the CONFIG=qemu build (*), but you beat me to it
> with the KVM emulation code ;)
> 
> (*) If you look at the iPXE commit that added them, they are a
> workaround for a Tivoli VMM bug; i.e., irrelevant for QEMU/KVM guests.
> 
> ... Actually, those iPXE patches that conditionalize FXSAVE / FXRSTOR
> may still make sense -- we can rebuild iPXE, and bundle the refreshed
> binaries with QEMU v2.7.1, and swiftly at that. Whereas the KVM patches
> could take more time to propagate to users?... Not sure. What do you
> guys think?

This series won't get into 4.9, so it would take almost half a year
before the kernel trickles into experimental distros.  And updating
QEMU/iPXE isn't as dangerous as updating kernel, so I like the idea.

I am just tempted to drop a KVM patch with positive diffstat that fixes
something that doesn't really need fixing anymore. :)