From mboxrd@z Thu Jan 1 00:00:00 1970 From: "H. Peter Anvin" Subject: Re: [PATCH 1/2] x86: eliminate TS_XSAVE Date: Tue, 04 May 2010 11:24:02 -0700 Message-ID: <4BE06642.6080702@zytor.com> References: <1272812038-32484-1-git-send-email-avi@redhat.com> <1272812038-32484-2-git-send-email-avi@redhat.com> <4BDDBA18.3080909@redhat.com> <4BDF4407.8000503@zytor.com> <4BDFCF9F.3030600@redhat.com> <1272996927.2833.13.camel@sbs-t61.sc.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: Avi Kivity , Brian Gerst , "Cui, Dexuan" , Sheng Yang , Ingo Molnar , "linux-kernel@vger.kernel.org" , "kvm@vger.kernel.org" To: Suresh Siddha Return-path: In-Reply-To: <1272996927.2833.13.camel@sbs-t61.sc.intel.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 05/04/2010 11:15 AM, Suresh Siddha wrote: > On Tue, 2010-05-04 at 00:41 -0700, Avi Kivity wrote: >> On 05/04/2010 12:45 AM, H. Peter Anvin wrote: >>> >>>> I was trying to avoid a performance regression relative to the current >>>> code, as it appears that some care was taken to avoid the memory reference. >>>> >>>> I agree that it's probably negligible compared to the save/restore >>>> code. If the x86 maintainers agree as well, I'll replace it with >>>> cpu_has_xsave. >>>> >>>> >>> I asked Suresh to comment on this, since he wrote the original code. He >>> did confirm that the intent was to avoid a global memory reference. >>> >>> >> >> Ok, so you're happy with the patch as is? > > As use_xsave() is in the hot context switch path, I would like to go > with Avi's proposal. > I would tend to agree. Saving a likely cache miss in the hot context switch path is worthwhile. I would like to request one change, however. I would like to see the alternatives code to be: movb $0,reg movb $1,reg ... instead of using xor (which has to be padded with NOPs, which is of course pointless since the slot is a fixed size.) I would suggest using a byte-sized variable instead of a dword-size variable to save a few bytes, too. Once the jump label framework is integrated and has matured, I think we should consider using it to save the mov/test/jump. -hpa