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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>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Henry Wang <Henry.Wang@xxxxxxx>
- Date: Fri, 28 Oct 2022 02:56:35 +0000
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- Thread-topic: [PATCH for-4.17 v3 2/2] vpci: refuse BAR writes only if the BAR is mapped
Hi Roger,
> -----Original Message-----
> From: Roger Pau Monne <roger.pau@xxxxxxxxxx>
> Subject: [PATCH for-4.17 v3 2/2] vpci: refuse BAR writes only if the BAR is
> mapped
>
> 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>
Release-acked-by: Henry Wang <Henry.Wang@xxxxxxx>
Kind regards,
Henry
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