Re: [Xen-devel] [PATCH 3/3] Add HVMOP to map guest ram with p2m_ioreq_server to an ioreq server

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 23/03/16 09:22, Jan Beulich wrote:
>>>> On 22.03.16 at 18:51, <Paul.Durrant@xxxxxxxxxx> wrote:
>>> From: George Dunlap [mailto:george.dunlap@xxxxxxxxxx]
>>> Sent: 22 March 2016 17:27
>>> There's not much documentation in the code about how this is expected to
>>> be used.
>>>
>>> For instance, having separate flags seems to imply that you can
>>> independently select either read intercept, write intercept, or both;
>>> but [ept_]p2m_type_to_flags() seems to assume that READ_ACCESS implies
>>> WRITE_ACCESS.  Do you plan to implement them separately in the future?
>>> If not, would it be better to make the interface an enum instead?
>>>
>>> At very least it should be documented that READ_ACCESS implies
>>> WRITE_ACCESS.
>> That's not true. If WRITE_ACCESS has not been requested then writes are 
>> handled directly in Xen rather than being forwarded to the ioreq server. If 
>> h/w were to allow pages to be marked write-only then we wouldn't need to do 
>> that.
> At least on EPT iirc you can have write-only pages, due to there
> being both an R and a W bit.

W and WX pages in EPT cause a MISCONFIG.  All other combinations however
are usable.

~Andrew

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