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

[Jeremi Piotrowski]

On Thu, Feb 16, 2023 at 10:32:28AM +0100, Juergen Gross wrote:
> On 13.02.23 12:39, Borislav Petkov wrote:
> >On Thu, Feb 09, 2023 at 08:22:14AM +0100, Juergen Gross wrote:
> >>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.
> >
> >s/cpuid/CPUID/g
> >
> >>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);
> >>  extern u8 mtrr_type_lookup(u64 addr, u64 end, u8 *uniform);
> >>  extern void mtrr_save_fixed_ranges(void *);
> >>  extern void mtrr_save_state(void);
> >>diff --git a/arch/x86/kernel/cpu/mtrr/generic.c b/arch/x86/kernel/cpu/mtrr/generic.c
> >>index ee09d359e08f..788bc16888a5 100644
> >>--- a/arch/x86/kernel/cpu/mtrr/generic.c
> >>+++ b/arch/x86/kernel/cpu/mtrr/generic.c
> >>@@ -240,6 +240,44 @@ static u8 mtrr_type_lookup_variable(u64 start, u64 end, u64 *partial_end,
> >>  	return mtrr_state.def_type;
> >>  }
> >>+/**
> >>+ * mtrr_overwrite_state - set fixed MTRR state
> >
> >fixed only? You pass in variable too...
> >
> >>+ *
> >>+ * Used to set MTRR state via different means (e.g. with data obtained from
> >>+ * a hypervisor).
> >>+ */
> >>+void mtrr_overwrite_state(struct mtrr_var_range *var, unsigned int num_var,
> >>+			  mtrr_type *fixed, mtrr_type def_type)
> >>+{
> >>+	unsigned int i;
> >>+
> >>+	if (boot_cpu_has(X86_FEATURE_MTRR))
> >
> >check_for_deprecated_apis: WARNING: arch/x86/kernel/cpu/mtrr/generic.c:254: Do not use boot_cpu_has() - use cpu_feature_enabled() instead.
> >
> >>+		return;
> >
> >So this here needs to check:
> >
> >	if (!cpu_feature_enabled(X86_FEATURE_HYPERVISOR) &&
> >	    !(cpu_feature_enabled(X86_FEATURE_SEV_SNP) ||
> >	      cpu_feature_enabled(X86_FEATURE_XENPV))) {
> >		WARN_ON_ONCE(1);
> >		return;
> >	}
> >
> >as we don't want this to be called somewhere or by something else.
> >
> >The SEV_SNP flag can be used from:
> >
> >https://lore.kernel.org/r/20221214194056.161492-14-michael.roth@xxxxxxx
> >
> >I'm assuming here HyperV SEV-SNP guests really do set that feature flag
> >(they better). We can expedite that patch ofc.
> 
> Is that flag _really_ meant to indicate we are running as a SEV-SNP guest?
> 
> Given that the referenced patch is part of the SEV-SNP host support series,
> I'm inclined to suspect it won't be set for sure in HyperV SEV-SNP guests.
> And who is setting it for KVM SEV-SNP guests?
> 
> 
> Juergen

Initially both guest and host side have X86_FEATURE_SEV_SNP, but
early_detect_mem_encrypt() clears it for the guest side.

You would want cc_platform_has(CC_ATTR_GUEST_SEV_SNP), but:

* there are two kinds of Hyper-V SEV-SNP VMs: "unenlightened" (vTOM+paravisor)
  and "enlightened"
* the kernel currently supports the "unenlightened" kind which do not set
  CC_ATTR_GUEST_SEV_SNP (because it implies codepaths which do not apply to
  vTOM mode)
* the patchset that adds support for "enlightened" Hyper-V SEV-SNP VMs sets
  CC_ATTR_GUEST_SEV_SNP [1]

The closest you can get is:

  cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT) && hypervisor_is_type(X86_HYPER_MS_HYPERV)

but that would soon cover TDX guests too so <shrug>...

[1]: https://lore.kernel.org/lkml/1673559753-94403-8-git-send-email-mikelley@xxxxxxxxxxxxx/