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

[Jason Andryuk <jandryuk@gmail.com>]

On Mon, Jan 20, 2020 at 12:18 PM Roger Pau Monné <roger.pau@citrix.com> wrote:
>
> On Mon, Jan 20, 2020 at 05:10:33PM +0100, Jan Beulich wrote:
> > On 17.01.2020 12:08, Roger Pau Monne wrote:
> > > When placing memory BARs with sizes smaller than 4K multiple memory
> > > BARs can end up mapped to the same guest physical address, and thus
> > > won't work correctly.
> >
> > Thinking about it again, aren't you fixing one possible case by
> > breaking the opposite one: What you fix is when the two distinct
> > BARs (of the same or different devices) map to distinct MFNs
> > (which would have required a single GFN to map to both of these
> > MFNs). But don't you, at the same time, break the case of two
> > BARs (perhaps, but not necessarily, of the same physical device)
> > mapping both to the same MFN, i.e. requiring to have two distinct
> > GFNs map to one MFN? (At least for the moment I can't see a way
> > for hvmloader to distinguish the two cases.)
>
> IMO we should force all BARs to be page-isolated by dom0 (since Xen
> doesn't have the knowledge of doing so), but I don't see the issue
> with having different gfns pointing to the same mfn. Is that a
> limitation of paging? I think you can map a grant multiple times into
> different gfns, which achieves the same AFAICT.

BARs on a shared MFN would be a problem since the second BAR would be
at an offset into the page.  Meanwhile the guest's view of the BAR
would be at offset 0 of the page.

But I agree with Roger that we basically need page alignment for all
pass-through memory BARs.  With my limited test hardware, all the PCI
memory BARs are page aligned in dom0.  So it was only the guest
addresses that needed alignment.

Regards,
Jason

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