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Re: [Xen-devel] [PATCH v2] x86emul: don't read mask register on AVX512F-incapable platforms
- To: Jan Beulich <JBeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Mon, 1 Apr 2019 15:14:17 +0100
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- Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
- Delivery-date: Mon, 01 Apr 2019 14:25:36 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 29/03/2019 10:56, Jan Beulich wrote:
>>>> On 29.03.19 at 11:02, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 29/03/2019 09:36, Jan Beulich wrote:
>>> I'd like to put up the other option then: Rather than using
>>> _get_fpu() (and in particular the read_xcr() and read_cr() hooks)
>>> we could read the real XCR0 here. After all we issue the KMOV not
>>> because the guest has specified it, but because we need the value
>>> of the register for correct fault suppression emulation.
>> True, and that would be rather smaller and less invasive than
>> deliberately squashing the other side effects of get_fpu()
> Hmm, I've tried to do this, but this is more complicated: CR0.TS
> may be set, in which case we need to invoke the get_fpu() hook
> to get it cleared with appropriate bookkeeping. I don't think it's
> worth further complicating the code by invoking the hook _only_
> in that case. So I guess we better stick to v2.
Oh ok. That does complicated things. Lets just use the existing
infrastructure, even if it is rather heavyweight.
>
> Which makes me come back to your request to drop the
> cpu_has_avx512f part of the condition: Right now the fuzzer
> uses emul_test_read_xcr() instead of actually fuzzing the
> value. Once it does, would we expect it to never set any bits
> in the returned value that aren't set in hardware, but could
> in principle be set based on (real) CPUID output? In that case
> I could agree to remove the extra condition.
I don't see how we could ever emulate with a (v)xcr0 different to a
legitimate value in hardware, as the stubs would #UD.
I also don't see how the userspace tools could ever test with a value
other than what it can see in xgetbv, because only the kernel gets to
choose %xcr0. Even with faking up a smaller xcr0, you'd end up with
instructions which should fault but don't.
~Andrew
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