linux-kernel - Re: [PATCH AUTOSEL 5.16 07/28] x86/kvm/fpu: Limit guest user

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Date:   Tue, 7 Jun 2022 14:54:14 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Sasha Levin <sashal@...nel.org>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, Leonardo Bras <leobras@...hat.com>,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org,
        chang.seok.bae@...el.com, luto@...nel.org, kvm@...r.kernel.org,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH AUTOSEL 5.16 07/28] x86/kvm/fpu: Limit guest
 user_xfeatures to supported bits of XCR0

On 6/6/22 23:27, Peter Xu wrote:
> On Mon, Jun 06, 2022 at 06:18:12PM +0200, Paolo Bonzini wrote:
>>> However there seems to be something missing at least to me, on why it'll
>>> fail a migration from 5.15 (without this patch) to 5.18 (with this patch).
>>> In my test case, user_xfeatures will be 0x7 (FP|SSE|YMM) if without this
>>> patch, but 0x0 if with it.
>>
>> What CPU model are you using for the VM?
> 
> I didn't specify it, assuming it's qemu64 with no extra parameters.

Ok, so indeed it lacks AVX and this patch can have an effect.

>> For example, if the source lacks this patch but the destination has it,
>> the source will transmit YMM registers, but the destination will fail to
>> set them if they are not available for the selected CPU model.
>>
>> See the commit message: "As a bonus, it will also fail if userspace tries to
>> set fpu features (with the KVM_SET_XSAVE ioctl) that are not compatible to
>> the guest configuration.  Such features will never be returned by
>> KVM_GET_XSAVE or KVM_GET_XSAVE2."
> 
> IIUC you meant we should have failed KVM_SET_XSAVE when they're not aligned
> (probably by failing validate_user_xstate_header when checking against the
> user_xfeatures on dest host). But that's probably not my case, because here
> KVM_SET_XSAVE succeeded, it's just that the guest gets a double fault after
> the precopy migration completes (or for postcopy when the switchover is
> done).

Difficult to say what's happening without seeing at least the guest code 
around the double fault (above you said "fail a migration" and I thought 
that was a different scenario than the double fault), and possibly which 
was the first exception that contributed to the double fault.

Paolo