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Re: [PATCH for-4.17 v3 2/2] vpci: refuse BAR writes only if the BAR is mapped
- To: Roger Pau Monne <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 27 Oct 2022 16:31:43 +0200
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- Cc: Henry.Wang@xxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Thu, 27 Oct 2022 14:31:51 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 27.10.2022 15:23, Roger Pau Monne wrote:
> Writes to the BARs are ignored if memory decoding is enabled for the
> device, and the same happen with ROM BARs if the write is an attempt
> to change the position of the BAR without disabling it first.
>
> The reason of ignoring such writes is a limitation in Xen, as it would
> need to unmap the BAR, change the address, and remap the BAR at the
> new position, which the current logic doesn't support.
>
> Some devices however seem to (wrongly) have the memory decoding bit
> hardcoded to enabled, and attempts to disable it don't get reflected
> on the command register.
>
> This causes issues for well behaved domains that disable memory
> decoding and then try to size the BARs, as vPCI will think memory
> decoding is still enabled and ignore the write.
>
> Since vPCI doesn't explicitly care about whether the memory decoding
> bit is disabled as long as the BAR is not mapped in the domain p2m use
> the information in the vpci_bar to check whether the BAR is mapped,
> and refuse writes only based on that information. This workarounds
> the issue, and allows domains to size and reposition the BARs properly.
>
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
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