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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

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.

Roger.

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel