Re: [PATCH] x86/CPU: don't hard-code MTRR availability

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.03.2025 09:21, Roger Pau Monné wrote:
> On Tue, Mar 25, 2025 at 08:18:11AM +0100, Jan Beulich wrote:
>> In particular if we're running virtualized, the underlying hypervisor
>> (which may be another Xen) may not surface MTRRs, and offer PAT only.
> 
> At least for Xen, I think we offer MTRR uniformly, even on PVH
> guests?

By default we do, but we discussed the option of offering PAT-only environments
beyond leaving it open whether people disabling MTRR support in their guest
config are outside of supported terrain.

>> Fixes: 5a281883cdc3 ("Hardcode many cpu features for x86/64 -- we know 
>> 64-bit")
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> 
> My main concern is whether the !mtrr path is still functional.  Have
> you tried booting the resulting hypervisor with MTRR masked on CPUID?
> 
> (or alternatively short-circuiting cpu_has_mtrr == 0?)

I didn't think this would be a prereq here. If we boot in an environment truly
lacking MTRR, we'll crash on the first MTRR MSR access - none of those accesses
use the safe accessors. Since you asked, I tried booting with "cpuid=no-mtrr".
As I'm doing this on a system with console, all I can say is that it takes
minutes to reach the point where we'd start setting up Dom0 (which itself then
takes so long that I timed out waiting for it to make progress), due to all
screen output becoming unbelievably slow after AP bringup. Surely something's
screwed somewhere, as VRAM accesses being slow (or fast) shouldn't depend on AP
bringup having completed. I actually suspect it's not just VRAM accesses which
are slow, but that we're left running in uncachable mode altogether for whatever
reason.

What this maybe useful for is to figure out the reason of "Platform timer
appears to have unexpectedly wrapped <N> times", which I saw appear once.

Given this, I'm actually uncertain whether it is legitimate then to take your
ack.

Jan