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Date:   Mon, 6 Jun 2022 17:27:38 -0400
From:   Peter Xu <peterx@redhat.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Sasha Levin <sashal@kernel.org>, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org, Leonardo Bras <leobras@redhat.com>,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        dave.hansen@linux.intel.com, x86@kernel.org,
        chang.seok.bae@intel.com, luto@kernel.org, kvm@vger.kernel.org,
        Sean Christopherson <seanjc@google.com>
Subject: Re: [PATCH AUTOSEL 5.16 07/28] x86/kvm/fpu: Limit guest
 user_xfeatures to supported bits of XCR0
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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.

I just tried two other options with: (1) -cpu host, and (2) -cpu Haswell
(the choice of Haswell was really random..), with the same 5.15->5.18
migration scenario, both of them will not trigger the same guest kernel
crash.  Only qemu64 will.

Both hosts have Intel(R) Xeon(R) CPU E5-2630 v4 @ 2.20GHz.

> 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).

Thanks,

-- 
Peter Xu