From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [CFT PATCH 2/2] KVM: x86: support XSAVES usage in the host Date: Mon, 24 Nov 2014 11:07:11 +0100 Message-ID: <5473034F.7030408@redhat.com> References: <1416594678-13011-1-git-send-email-pbonzini@redhat.com> <1416594678-13011-3-git-send-email-pbonzini@redhat.com> <20141124021009.GB27849@kernel> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 8bit Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Fenghua Yu , stable@vger.kernel.org, Nadav Amit , Andy Lutomirski To: Wanpeng Li Return-path: In-Reply-To: <20141124021009.GB27849@kernel> Sender: stable-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 24/11/2014 03:10, Wanpeng Li wrote: > Hi Paolo, > On Fri, Nov 21, 2014 at 07:31:18PM +0100, Paolo Bonzini wrote: > [...] >> + u64 feature = valid & -valid; >> + int index = fls64(feature) - 1; >> + void *src = get_xsave_addr(xsave, feature); >> + >> + if (src) { >> + u32 size, offset, ecx, edx; >> + cpuid_count(XSTATE_CPUID, index, >> + &size, &offset, &ecx, &edx); >> + memcpy(dest + offset, src, size); > > The offset you get is still for compact format No, it's not, or all old software using XSAVE/XRSTOR would be broken. The code in arch/x86/kernel/xsave.c agrees with me; compacted offsets (xsave_comp_offsets) are computed by summing sizes, while non-compacted offsets (xsave_offsets) come for CPUID. > , so you almost convert compat > format to compat format instead of convert compact format to standard format. > In addition, I think convert standard format to compact format should be > implemented in put path. If I do that, userspace is broken because it expects standard format. Hence, passing XSAVE data to userspace in compact format can be done, but has to be guarded by an explicitly enabled capability (using KVM_ENABLE_CAP). I do not think that's useful, since no supervisor-specific states are defined yet, and anyway they can be passed using KVM_GET/SET_MSR because this is not a fast path. Paolo