Re: [Xen-devel] [PATCH]Do some checks and settings of CR0according to VMX capability MSRs

[Keir Fraser <keir@xxxxxxxxxxxxx>]

Working correctly with arbitrary settings of the capabilities MSRs is clearly impossible. The real question is: what are the most likely sorts of changes in future processors? My assumption is that capabilities will increase and hence the constraints indicated by the capability MSRs will decrease. In which case we can evolve the capability-checking code over time, as Xen develops the ability to make use of new processor features (e.g., if a future version of VMX supports paged real mode). I don’t think it is likely that constraints will increase in future (e.g., bits must be set or clear that could previously be set as you like; or even that a bit that was forced one way in one version of VMX must be set the other way in a later version). Trying to support that gracefully and well would be a total pain, and to what end?

 -- Keir

On 1/8/07 05:50, "Li, Xin B" <xin.b.li@xxxxxxxxx> wrote:

good point, and this is a two side issue, one is hardware capability, the other is software capability, maybe a better way is to have another CR0 value expected by software, and use both of hardware and software expected value to get the effiective one?
-Xin

 

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From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx  [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Keir  Fraser
Sent: Friday, July 27, 2007 4:19 PM
To: Liu, Eric  E; xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-devel] [PATCH]Do  some checks and settings of CR0according to VMX capability  MSRs

 
I agree with some aspects of this patch but not  others. For example, not explicitly including PG and PE in guest-mode cr0  value is a bad idea. If a future processor does support e.g., paged real mode  then it won’t magically be the case that old Xen will know how to handle that.  Currently we expect and require a VMX guest always to run with  CR0.PE==1.

I’ll pull out the bits of the patch that I  like.

 -- Keir

On 27/7/07 09:03, "Liu, Eric E"  <eric.e.liu@xxxxxxxxx> wrote:

 

According to SDM Vol 3B 19.8 Software should consult  the VMX capability MSRs to determine how bits
in CR0 are set, VMXON  fails if any of these bits contains an unsupported value. And according to  
SDM Vol 3A 2.5, 3B 21.3 and 2A MOV-MOV to/from Control Registers,  setting upper 32 bits of CR0
results in a general-protection exception  and setting the reserved bits in lower 32 bits of CR0 are ignored .
In  accordance with above-mentioned, the patch is attached to do some checks and  settings of CR0.

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