Re: [PATCH] xen/pt: Avoid initializing BARs from the host ones

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

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?

> 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.

Thanks, Roger.