Re: [Xen-devel] [PATCH] x86/boot: Don't explicitly map the VGA region as UC-

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

On Thu, Sep 05, 2019 at 08:04:18PM +0100, Andrew Cooper wrote:
> All 64-bit capable processors use PAT, and with PAT, it is explicitly
> permitted to have mappings with a cacheability different to MTRRs.
> 
> On a native system with a real legacy VGA region, MTRRs will cause the region
> to be UC-.  When booting virtualised, this range may be RAM and not a legacy
> VGA region, at which point it wants to be WB.
> 
> Use WB mapping in the pagetables, such that in systems without a legacy VGA
> region, the RAM between 0xa0000 and 0xc0000 doesn't become unnecesserily UC-.
> However, we still must use small pages to avoid the undefined behaviour caused
> by superpages crossing MTRRs of different cacheability.
> 
> While adjusting the pagetable construction, switch from pfn to idx for
> consistency with all the other construction logic.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Wei Liu <wl@xxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> 
> As a future optimisation, Xen could rewrite l2_identmap with a superpage if it
> finds that MTRRs are disabled.  However, this probably needs to wait until Xen
> no longer unconditionally assumes a legacy VGA region to be present in the
> e820 if it finds something other than a hole

Is that still the case? I remember dealing with it when working on the
shim, and I thought the code to unconditionally set a hole in
0xa0000-0xc0000 was removed.

Roger.

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