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

[Jan Beulich <jbeulich@xxxxxxxx>]

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?

> IMO this patch is an improvement over the current state, and we can
> always do further improvements afterwards.

As said, to me it looks as if it was breaking one case in order to fix
another. If I'm not wrong with this, I don't see how the patch is an
improvement.

Jan

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