From: Juergen Gross <jgross@xxxxxxxx> Sent: Monday, February 6, 2023 11:29 PM
This series tries to fix the rather special case of PAT being available
without having MTRRs (either due to CONFIG_MTRR being not set, or
because the feature has been disabled e.g. by a hypervisor).
The main use cases are Xen PV guests and SEV-SNP guests running under
Hyper-V.
Patch 2 seems to be a little hacky, as it special cases only
memtype_reserve() and memtype_free(), but OTOH this doesn't seem to
be worse than in previous days, where PAT was disabled when MTRRs
haven't been available.
My tests with Xen didn't show any problems, but I'm rather sure I
couldn't cover all corner cases.
I tested this patch set with Hyper-V SEV-SNP guests, and ioremap_cache()
is correctly mapping as WB.
As an observation, with commit 90b926e68f50 it was nice to have
the memtype entries created. I could check for any unexpected
mappings in /sys/kernel/debug/x86/pat_memtype_list. With this patch
set, we're back to not creating those entries.
Michael
The only cleaner solution I could think of would be to introduce MTRR
read-only access. It would theoretically be possible to get the actual
MTRR contents for the variable MTRRs from Xen, but I'm not sure this
is really the way to go.
For the SEV-SNP case with Hyper-V I guess such a read-only mode could
be rather simple, but I'm really not sure this would cover all needed
corner cases (I'd basically say always "WB" in that case).
Attachment:
OpenPGP_0xB0DE9DD628BF132F.asc
Description: OpenPGP public key
Attachment:
OpenPGP_signature
Description: OpenPGP digital signature