Re: [Xen-devel] [PATCH v2 1/2] arm/mem_access: adjust check_and_get_page to not rely on current

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 13/12/16 18:41, Julien Grall wrote:
> Hi Andrew,
>
> On 13/12/16 18:28, Andrew Cooper wrote:
>> On 12/12/16 23:47, Tamas K Lengyel wrote:
>>>
>>>> It works on x86 because, IIRC, you do a software page table walking.
>>>> Although, I don't think you have particular write/read access
>>>> checking on
>>>> x86.
>>> I don't recall there being any software page walking being involved on
>>> x86. Why would that be needed? On x86 we get the guest physical
>>> address recorded by the CPU automatically. AFAIK in case the pagetable
>>> was unaccessible for the translation of a VA, we would get an event
>>> with the pagetable's PA and the type of event that generated it (ie.
>>> reading during translation).
>>
>> x86 provides no mechanism to have hardware translate an address in a
>> separate context.  Therefore, all translations which are necessary need
>> to be done in hardware.
>
> I guess you meant "need to be done in sofware"?

I did.  Sorry for the confusion.

>
>>
>> Newer versions of VT-x/SVM may provide additional information on a
>> vmexit, which include a guest physical address relevant to the the
>> reason for the vmexit.
>>
>> Xen will attempt to proactively use this information to avoid a software
>> pagewalk, but can always fall back to the software method if needs be.
>
> That is a good idea, but may bring some issue with memacces as we
> currently have on ARM.

Why would a software-only approach have problem on ARM?  Slow certainly,
but it should function correctly.

>
>>
>>
>> I presume ARM has always relied on hardware support for translation, and
>> has no software translation support?  I presume therefore that
>> translations only work when in current context?
>
> That's correct. ARM provided hardware support for most of translation
> from the start. But it relies on the guest page table to be accessible
> (e.g there is no memory restriction in stage-2).

Ok, so ARM provides an instruction to translate an arbitrary virtual
address to guest physical.  How does this work in the context of Xen, or
can hardware follow the currently-active virtualisation state to find
the guest pagetables?  Or does it rely on information in the TLB?

> Note that ARM does not provide any hardware instruction to translate
> an IPA (guest physical address) to a PA. So we have a walker there.
>
> We also a walk for debugging guest page table (though only when it is
> using LPAE). I guess it could be re-used in the case where it is not
> possible to do it in hardware. Although, it would be rewritten to make
> it safe.

This sounds like the kind of thing which would be generally useful,
although I'd like to understand the problem better before making
suggestions.

~Andrew

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel