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

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Date:   Mon, 6 Jun 2022 18:18:12 +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 6/3/22 20:40, Peter Xu wrote:
> I'm not really sure whether this is a bug or by design - do we require this
> patch to be applied to all stable branches to make the guest not crash
> after migration, or it is unexpected?

Yes, we do, though the only reported bug was for PKRU.

> 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?  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."

Paolo