From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [CFT PATCH v2 2/2] KVM: x86: support XSAVES usage in the host Date: Wed, 26 Nov 2014 14:57:04 +0100 Message-ID: <5475DC30.4010104@redhat.com> References: <1416847414-22253-1-git-send-email-pbonzini@redhat.com> <1416847414-22253-3-git-send-email-pbonzini@redhat.com> <20141126120753.GA31982@potion.redhat.com> <5475D20A.90505@redhat.com> <20141126135322.GA4887@potion.brq.redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, wanpeng.li@linux.intel.com, namit@cs.technion.ac.il, hpa@linux.intel.com, Fenghua Yu To: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= Return-path: In-Reply-To: <20141126135322.GA4887@potion.brq.redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 26/11/2014 14:53, Radim Kr=C4=8Dm=C3=A1=C5=99 wrote: >>> > > get_xsave =3D native_xrstor(guest_xsave); xsave(aligned_usersp= ace_buffer) >>> > > set_xsave =3D xrstor(aligned_userspace_buffer); native_xsave(g= uest_xsave) >>> > >=20 >>> > > Could that work? >> >=20 >> > It could, though it is more like >> >=20 >> > get_fpu() >> > native_xrstor(guest_xsave) >> > xsave(buffer) >> > put_fpu() >> >=20 >> > and vice versa. Also, the userspace buffer is mos likely not alig= ned, >> > so you need some kind of bounce buffer. It can be done if the CPU= ID >> > turns out to be a bottleneck, apart from that it'd most likely be = slower. > Yeah, it was mostly making this code more future-proof ... it is easi= er > to convince xsave.h to export its structures if CPUID is the problem. > (I still see some hope for Linux, so performance isn't my primary goa= l.) >=20 > I'm quite interested in CPUID now though, so I'll try to benchmark it= , > someday. I'm not sure what is more future proof. :) I wonder if native_xrstor could be a problem the day XRSTORS actually sets/restores MSRs as the processor documentation promises. We do not need that to pass them to userspace via KVM_GET/SET_XSAVE because we have KVM_GET/SET_MSR for that, but it may cause problems if get_xsave uses XRSTORS and thus sets the MSRs to unanticipated values. Difficult to say without more information on Intel's plans. Paolo