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Re: [PATCH] xen/pt: Avoid initializing BARs from the host ones
- To: Roger Pau Monne <roger.pau@xxxxxxxxxx>
- From: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
- Date: Mon, 9 May 2022 10:39:32 +0000
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Anthony Perard <anthony.perard@xxxxxxxxxx>, Paul Durrant <paul@xxxxxxx>, "qemu-devel@xxxxxxxxxx" <qemu-devel@xxxxxxxxxx>
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- Thread-topic: [PATCH] xen/pt: Avoid initializing BARs from the host ones
> From: Roger Pau Monne <roger.pau@xxxxxxxxxx>
> Sent: Monday, May 9, 2022 10:28 AM
> To: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx <xen-devel@xxxxxxxxxxxxxxxxxxxx>; Stefano
> Stabellini <sstabellini@xxxxxxxxxx>; Anthony Perard
> <anthony.perard@xxxxxxxxxx>; Paul Durrant <paul@xxxxxxx>;
> qemu-devel@xxxxxxxxxx <qemu-devel@xxxxxxxxxx>
> Subject: Re: [PATCH] xen/pt: Avoid initializing BARs from the host ones
>
> On Wed, Apr 27, 2022 at 05:38:12PM +0100, Ross Lagerwall via wrote:
> > The BAR emulated register definition does not set emu_mask because it
> > varies depending on bar_flag. If emu_mask is not set, then the BAR is
> > initialized based on the host value which causes the BAR to be initially
> > mapped at whatever value the host device was using. Although it does
> > eventually get mapped at the correct location, it causes unnecessary
> > mapping/unmappings.
>
> Would it be possible to just unset the memory decoding bit in the
> command register if it's set?
I don't think that would be sufficient since AFAICT qemu does not read that
bit so will still call into Xen to update memory mappings, etc.
>
> > To fix this, initialize a per-register emu_mask in XenPTReg from the
> > initial value in XenPTRegInfo and then let the register's init() function
> > set/modify the emu_mask if necessary. Update the code to use emu_mask
> > in XenPTReg consistently and rename the existing emu_mask in
> > XenPTRegInfo to emu_mask_init to help with refactoring.
>
> Iff we really need this refactoring it would better be done in a
> separate (pre)patch, so it's clear what's the fix and what are
> refactoring changes.
>
> I also wonder whether why it isn't enough to set emu_mask in
> xen_pt_bar_reg_init(), without having to introduce a new field.
>
That doesn't work since emu_mask is part of xen_pt_emu_reg_header0 which
is static and is not defined per passthrough device so having multiple
passthrough devices would cause issues.
I have an idea to achieve the same result with less code churn so I'll
try it and send an updated patch.
Ross
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