RE: [PATCH v2 2/8] x86/mtrr: support setting MTRR state for software defined MTRRs

[Michael Kelley (LINUX)]

From: Juergen Gross <jgross@xxxxxxxx> Sent: Sunday, February 12, 2023 10:27 PM
> 
> On 13.02.23 02:07, Michael Kelley (LINUX) wrote:
> > From: Juergen Gross <jgross@xxxxxxxx> Sent: Wednesday, February 8, 2023 11:22 PM
> >>
> >> When running virtualized, MTRR access can be reduced (e.g. in Xen PV
> >> guests or when running as a SEV-SNP guest under Hyper-V). Typically
> >> the hypervisor will reset the MTRR feature in cpuid data, resulting
> >> in no MTRR memory type information being available for the kernel.
> >>
> >> This has turned out to result in problems:
> >>
> >> - Hyper-V SEV-SNP guests using uncached mappings where they shouldn't
> >> - Xen PV dom0 mapping memory as WB which should be UC- instead
> >>
> >> Solve those problems by supporting to set a fixed MTRR state,
> >> overwriting the empty state used today. In case such a state has been
> >> set, don't call get_mtrr_state() in mtrr_bp_init(). The set state
> >> will only be used by mtrr_type_lookup(), as in all other cases
> >> mtrr_enabled() is being checked, which will return false. Accept the
> >> overwrite call only in case of MTRRs being disabled in cpuid.
> >>
> >> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> >> ---
> >> V2:
> >> - new patch
> >> ---
> >>   arch/x86/include/asm/mtrr.h        |  2 ++
> >>   arch/x86/kernel/cpu/mtrr/generic.c | 38 ++++++++++++++++++++++++++++++
> >>   arch/x86/kernel/cpu/mtrr/mtrr.c    |  9 +++++++
> >>   3 files changed, 49 insertions(+)
> >>
> >> diff --git a/arch/x86/include/asm/mtrr.h b/arch/x86/include/asm/mtrr.h
> >> index f0eeaf6e5f5f..0b8f51d683dc 100644
> >> --- a/arch/x86/include/asm/mtrr.h
> >> +++ b/arch/x86/include/asm/mtrr.h
> >> @@ -31,6 +31,8 @@
> >>    */
> >>   # ifdef CONFIG_MTRR
> >>   void mtrr_bp_init(void);
> >> +void mtrr_overwrite_state(struct mtrr_var_range *var, unsigned int num_var,
> >> +			  mtrr_type *fixed, mtrr_type def_type);
> >
> > Could you add a stub for the !CONFIG_MTRR case?  Then the
> > #ifdef CONFIG_MTRR could be removed in Patch 3 of this series.
> 
> I was on the edge whether to add a stub. The Xen use case strongly
> suggests that the code wants to be inside an #ifdef, while the Hyper-V
> case is so simple, that it would benefit from the stub. As there was
> another #ifdef just above the added code in mshyperv.c I believed it
> would be fine without a stub. As you seem to like it better with the
> stub, I can add it.
> 

Thanks.  And that other #ifdef is going away soon ...

Michael