Re: [Xen-devel] [PATCH v2] x86/hvmloader: round up memory BAR size to 4K

[Jan Beulich <jbeulich@xxxxxxxx>]

On 21.01.2020 16:57, Roger Pau Monné wrote:
> On Tue, Jan 21, 2020 at 11:43:58AM +0100, Jan Beulich wrote:
>> On 21.01.2020 11:29, Roger Pau Monné wrote:
>>> So I'm not sure how to progress with this patch, are we fine with
>>> those limitations?
>>
>> I'm afraid this depends on ...
>>
>>> As I said, Xen hasn't got enough knowledge to correctly isolate the
>>> BARs, and hence we have to rely on dom0 DTRT. We could add checks in
>>> Xen to make sure no BARs share a page, but it's a non-trivial amount
>>> of scanning and sizing each possible BAR on the system.
>>
>> ... whether Dom0 actually "DTRT", which in turn is complicated by there
>> not being a specific Dom0 kernel incarnation to check against. Perhaps
>> rather than having Xen check _all_ BARs, Xen or the tool stack could
>> check BARs of devices about to be handed to a guest? Perhaps we need to
>> pass auxiliary information to hvmloader to be able to judge whether a
>> BAR shares a page with another one? Perhaps there also needs to be a
>> way for hvmloader to know what offset into a page has to be maintained
>> for any particular BAR, as follows from Jason's recent reply?
> 
> Linux has an option to force resource alignment (as reported by
> Jason), maybe we could force all BARs to be aligned to page size in
> order to be passed through?
> 
> That would make it easier to check (as Xen/Qemu would only need to
> assert that the BAR address is aligned), and won't require much extra
> work in Xen apart from the check itself.
> 
> Do you think this would be an acceptable solution?

In principle yes, but there are loose ends:
- What do you mean by "we could force"? We have no control over the
  Dom0 kernel.
- What about non-Linux Dom0?

Also, apart from extra resource (address space) consumption, what's
the point of forcing a single device's BARs to separate pages? (I'm
assuming here that hvmloader would have a way to know of the
potentially resulting non-zero offsets into a page. And I'm still
puzzled that the lack thereof hasn't been reported as a bug by
anyone, afaik.)

Jan

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