Re: [Xen-devel] XSA-60 solutions

["Liu, Jinsong" <jinsong.liu@xxxxxxxxx>]

Jan Beulich wrote:
>>>> On 07.10.13 at 18:03, "Liu, Jinsong" <jinsong.liu@xxxxxxxxx> wrote:
>> Jan Beulich wrote:
>>>>>> On 07.10.13 at 16:29, "Liu, Jinsong" <jinsong.liu@xxxxxxxxx>
>>>>>> wrote: Any comments/suggestions?
>>> 
>>> As pointed out in earlier private conversation, I think that the
>>> dual table approach would be preferable if it can be made work.
>> 
>> No, solution 2 has an important update compared with our private
>> conversation before:
>> The old solution 2 is, Xen do nothing for guest cr0.cd setting.
>> The new solution 2 in this email is, Xen set IA32_PAT fields as UC
>> when needed (that means, only when guest work with vt-d but vt-d
>> non-snooped, since under this case h/w can not guarantee cache
>> coherency), so that all guest memory type are UC. For other cases
>> (when guest work w/o vt-d, and when guest work with vt-d but vt-d
>> snooped), Xen can still do nothing for guest cr0.cd setting (since
>> h/w has guaranteed cache coherency, otherwise Xen doesn't dare to do
>> what it does now -- it set iPAT bit as 1, totally ignoring guest
>> memory type point of view).  
>> 
>> Compared with Dual-EPT solution, it's much simpler (only need
>> IA32_PAT MSR emulation under rare case) and avoid the inconsistency
>> issue between 2 EPT tables.
> 
> Right - as long as PAT is guaranteed to only be used for guest
> induced memory accesses, that sounds like a viable yet not too
> intrusive solution. Short of anyone else having an opinion here,
> why don't you start drafting a patch for this?
> 
> Jan

That's great! We will draft patch.

Another point is, for shadow, currently Xen drop all shadows so that any new 
ones would be created on demand with UC. It works safely for XSA-60, but seems 
not good enough. We can still use IA32_PAT to solve it, not touch shadow page 
table itself.

One thing I need more confirm from AMD side: iirc, AMD IOMMU always works with 
snooped so cache cohernecy has been guaranteed, so no need do anything for 
guest cr0.cd setting, no matter shadow or NPT, right? If yes, most code could 
be done w/ Intel specific.

Thanks,
Jinsong
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