From: Paolo Bonzini <pbonzini@redhat.com>
To: Nadav Amit <nadav.amit@gmail.com>
Cc: kvm list <kvm@vger.kernel.org>
Subject: Re: Problem with KVM when using XSAVES in host
Date: Sun, 14 Dec 2014 16:55:47 +0100 [thread overview]
Message-ID: <548DB303.9070707@redhat.com> (raw)
In-Reply-To: <3F0E7E55-133E-42E7-9B5B-AAEDD259B992@gmail.com>
On 14/12/2014 15:31, Nadav Amit wrote:
> This behaviour hurts the “equivalence property” - the VM does not behave as
> bare-metal system. Moreover, it may hurt the VM performance if the VM uses
> XSAVEOPT (and not XSAVES), has MXCSR with value different than the reset
> value of 1F80H and has all SSE registers set to zero. In such case, the VM
> would save/restore SSE registers unnecessarily. I don’t know whether such
> scenario happens in real workloads.
>
> tl;dr - hypervisors which use XSAVES (and XSAVEC) mess the VM state and may
> hurt VM performance. Perhaps KVM should use XSAVE/XSAVEOPT instead.
Unfortunately KVM uses whatever the kernel uses. KVM does not
save/restore itself the FPU state.
The same problem above can happen if the kernel uses XSAVES/XRSTORS and
userspace uses XSAVEOPT. So it's a processor bug.
Paolo
prev parent reply other threads:[~2014-12-14 15:55 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-12-14 14:31 Problem with KVM when using XSAVES in host Nadav Amit
2014-12-14 15:55 ` Paolo Bonzini [this message]
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