What is the reason for not simply clearing the MSR bitmap bits for these three MSRs, and rather handling the reads/writes in software? Jan _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
Probably because the existing code must remain anyway (since not all VMX CPUs support the MSR bitmap), and the benefit of avoiding the intercepts on CPUs that do support it would have entirely negligible performance benefit, since the MSRs are written only at start of day. We could add calls to avoid the intercepts anyway though, if you want to provide a patch. -- Keir On 6/7/07 13:41, "Jan Beulich" <jbeulich@novell.com> wrote:> What is the reason for not simply clearing the MSR bitmap bits for these three > MSRs, and rather handling the reads/writes in software? > > Jan > > > _______________________________________________ > Xen-devel mailing list > Xen-devel@lists.xensource.com > http://lists.xensource.com/xen-devel_______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
>What is the reason for not simply clearing the MSR bitmap bits >for these three MSRs, and rather handling the reads/writes in software?I haven''t checked those MSRs, the original concern is we need save/restore them when context switch, is this needed? -Xin _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
Since they are included in the VMCS, and read from there on save/restore, the vmexit appears to be unnecessary. -- Keir On 7/7/07 08:48, "Li, Xin B" <xin.b.li@intel.com> wrote:> >> What is the reason for not simply clearing the MSR bitmap bits >> for these three MSRs, and rather handling the reads/writes in software? > > I haven''t checked those MSRs, the original concern is we need > save/restore them when context switch, is this needed? > > -Xin > > _______________________________________________ > Xen-devel mailing list > Xen-devel@lists.xensource.com > http://lists.xensource.com/xen-devel_______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
I will send out the patch soon. -Xin>-----Original Message----- >From: Keir Fraser [mailto:Keir.Fraser@cl.cam.ac.uk] >Sent: Saturday, July 07, 2007 4:52 PM >To: Li, Xin B; Jan Beulich; xen-devel@lists.xensource.com >Subject: Re: [Xen-devel] VMX: emulated guest use SYSENTER MSRs > >Since they are included in the VMCS, and read from there on >save/restore, >the vmexit appears to be unnecessary. > > -- Keir > >On 7/7/07 08:48, "Li, Xin B" <xin.b.li@intel.com> wrote: > >> >>> What is the reason for not simply clearing the MSR bitmap bits >>> for these three MSRs, and rather handling the reads/writes >in software? >> >> I haven''t checked those MSRs, the original concern is we need >> save/restore them when context switch, is this needed? >> >> -Xin >> >> _______________________________________________ >> Xen-devel mailing list >> Xen-devel@lists.xensource.com >> http://lists.xensource.com/xen-devel >_______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
>>> "Li, Xin B" <xin.b.li@intel.com> 07.07.07 10:57 >>> >I will send out the patch soon.Not strictly necessary - I have this as part of a larger patch already. But if you beat me in getting this out, I''ll sync up with yours. Jan>-----Original Message----- >From: Keir Fraser [mailto:Keir.Fraser@cl.cam.ac.uk] >Sent: Saturday, July 07, 2007 4:52 PM >To: Li, Xin B; Jan Beulich; xen-devel@lists.xensource.com >Subject: Re: [Xen-devel] VMX: emulated guest use SYSENTER MSRs > >Since they are included in the VMCS, and read from there on >save/restore, >the vmexit appears to be unnecessary. > > -- Keir > >On 7/7/07 08:48, "Li, Xin B" <xin.b.li@intel.com> wrote: > >> >>> What is the reason for not simply clearing the MSR bitmap bits >>> for these three MSRs, and rather handling the reads/writes >in software? >> >> I haven''t checked those MSRs, the original concern is we need >> save/restore them when context switch, is this needed? >> >> -Xin >> >> _______________________________________________ >> Xen-devel mailing list >> Xen-devel@lists.xensource.com >> http://lists.xensource.com/xen-devel >_______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel