Re: [PATCH] x86/cpu: Add a VMX flag to enumerate 5-level EPT support to userspace

[Chao Gao]

On Tue, Jan 09, 2024 at 04:23:40PM -0800, Sean Christopherson wrote:
>Add a VMX flag in /proc/cpuinfo, ept_5level, so that userspace can query
>whether or not the CPU supports 5-level EPT paging.  EPT capabilities are
>enumerated via MSR, i.e. aren't accessible to userspace without help from
>the kernel, and knowing whether or not 5-level EPT is supported is sadly
>necessary for userspace to correctly configure KVM VMs.

This assumes procfs is enabled in Kconfig and userspace has permission to
access /proc/cpuinfo. But it isn't always true. So, I think it is better to
advertise max addressable GPA via KVM ioctls.

>
>When EPT is enabled, bits 51:49 of guest physical addresses are consumed
>if and only if 5-level EPT is enabled.  For CPUs with MAXPHYADDR > 48, KVM
>*can't* map all legal guest memory if 5-level EPT is unsupported, e.g.
>creating a VM with RAM (or anything that gets stuffed into KVM's memslots)
>above bit 48 will be completely broken.
>
>Having KVM enumerate guest.MAXPHYADDR=48 in this scenario doesn't work
>either, as architecturally guest accesses to illegal addresses generate
>RSVD #PF, i.e. advertising guest.MAXPHYADDR < host.MAXPHYADDR when EPT is
>enabled would also result in broken guests.  KVM does provide a knob,
>allow_smaller_maxphyaddr, to let userspace opt-in to such setups, but
>that support is firmly best-effort, i.e. not something KVM wants to force
>upon userspace.
>
>While it's decidedly odd for a CPU to support a 52-bit MAXPHYADDR but not
>5-level EPT, the combination is architecturally legal and such CPUs do
>exist (and can easily be "created" with nested virtualization).
>
>Reported-by: Yi Lai <yi1.lai@xxxxxxxxx>
>Cc: Tao Su <tao1.su@xxxxxxxxxxxxxxx>
>Cc: Xudong Hao <xudong.hao@xxxxxxxxx>
>Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
>---
>
>tip-tree folks, this is obviously not technically KVM code, but I'd like to
>take this through the KVM tree so that we can use the information to fix
>KVM selftests (hopefully this cycle).
>
> arch/x86/include/asm/vmxfeatures.h | 1 +
> arch/x86/kernel/cpu/feat_ctl.c     | 2 ++
> 2 files changed, 3 insertions(+)
>
>diff --git a/arch/x86/include/asm/vmxfeatures.h b/arch/x86/include/asm/vmxfeatures.h
>index c6a7eed03914..266daf5b5b84 100644
>--- a/arch/x86/include/asm/vmxfeatures.h
>+++ b/arch/x86/include/asm/vmxfeatures.h
>@@ -25,6 +25,7 @@
> #define VMX_FEATURE_EPT_EXECUTE_ONLY	( 0*32+ 17) /* "ept_x_only" EPT entries can be execute only */
> #define VMX_FEATURE_EPT_AD		( 0*32+ 18) /* EPT Accessed/Dirty bits */
> #define VMX_FEATURE_EPT_1GB		( 0*32+ 19) /* 1GB EPT pages */
>+#define VMX_FEATURE_EPT_5LEVEL		( 0*32+ 20) /* 5-level EPT paging */
> 
> /* Aggregated APIC features 24-27 */
> #define VMX_FEATURE_FLEXPRIORITY	( 0*32+ 24) /* TPR shadow + virt APIC */
>diff --git a/arch/x86/kernel/cpu/feat_ctl.c b/arch/x86/kernel/cpu/feat_ctl.c
>index 03851240c3e3..1640ae76548f 100644
>--- a/arch/x86/kernel/cpu/feat_ctl.c
>+++ b/arch/x86/kernel/cpu/feat_ctl.c
>@@ -72,6 +72,8 @@ static void init_vmx_capabilities(struct cpuinfo_x86 *c)
> 		c->vmx_capability[MISC_FEATURES] |= VMX_F(EPT_AD);
> 	if (ept & VMX_EPT_1GB_PAGE_BIT)
> 		c->vmx_capability[MISC_FEATURES] |= VMX_F(EPT_1GB);
>+	if (ept & VMX_EPT_PAGE_WALK_5_BIT)
>+		c->vmx_capability[MISC_FEATURES] |= VMX_F(EPT_5LEVEL);
> 
> 	/* Synthetic APIC features that are aggregates of multiple features. */
> 	if ((c->vmx_capability[PRIMARY_CTLS] & VMX_F(VIRTUAL_TPR)) &&
>
>base-commit: 1c6d984f523f67ecfad1083bb04c55d91977bb15
>-- 
>2.43.0.472.g3155946c3a-goog
>
>