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Re: [PATCH] x86/CPU: don't hard-code MTRR availability
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 27 Mar 2025 10:30:05 +0100
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Delivery-date: Thu, 27 Mar 2025 09:30:15 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 27.03.2025 10:15, Jan Beulich wrote:
> On 27.03.2025 09:21, Roger Pau Monné wrote:
>> 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.
I think found a trivial way to avoid this, and the change there imo makes sense
even if I can't explain this particular aspect of it. I'll submit that
independently.
Jan
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