From: Sheng Yang <sheng@linux.intel.com>
Subject: Re: [PATCH 1/7] KVM: VMX: Allocate MSR Bitmap for each vcpu
Date: Wed, 8 Oct 2008 18:41:30 +0800
Message-ID: <200810081841.30880.sheng@linux.intel.com>
References: <1223439478-26515-1-git-send-email-sheng@linux.intel.com> <1223439478-26515-2-git-send-email-sheng@linux.intel.com> <48EC87FE.3060006@redhat.com>
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Cc: kvm@vger.kernel.org
To: Avi Kivity <avi@redhat.com>
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On Wednesday 08 October 2008 18:14:22 Avi Kivity wrote:
> Sheng Yang wrote:
> > Separate msr_bitmap for each vcpu, prepared for guest PAT support.
>
> Why is this necessary?  True, it reduces the overhead of the guest
> reading and writing the PAT MSRs, but is such access frequent?
>
> I would think guests set the PAT once, and never change it later.

Yeah. In fact, I just think msr_bitmap for each vcpu would be done sooner or 
later, so get it done here. And it's natural to go with GUEST_PAT. And Xen 
use it for another purpose(DEBUGCTLMSR related, I haven't checked it through)
(Also svm.c in KVM use per-vcpu msr bitmap)

And without that, a callback should be implement to hook MSR write and update 
guest pat write for both vmx and svm, or we should update GUEST PAT every 
vmentry according to the vcpu->pat. Either way seems not that natural with 
GUEST_PAT support. 

However, if you think msr_bitmap for each vcpu is a waste, I'd like to add 
callbacks.

--
regards
Yang, Sheng