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Message-Id: <20240802151608.72896-2-mlevitsk@redhat.com>
Date: Fri, 2 Aug 2024 18:16:07 +0300
From: Maxim Levitsky <mlevitsk@...hat.com>
To: kvm@...r.kernel.org
Cc: Sean Christopherson <seanjc@...gle.com>,
linux-kernel@...r.kernel.org,
Borislav Petkov <bp@...en8.de>,
"H. Peter Anvin" <hpa@...or.com>,
Paolo Bonzini <pbonzini@...hat.com>,
Ingo Molnar <mingo@...hat.com>,
x86@...nel.org,
Thomas Gleixner <tglx@...utronix.de>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Maxim Levitsky <mlevitsk@...hat.com>,
Chao Gao <chao.gao@...el.com>
Subject: [PATCH v2 1/2] KVM: x86: relax canonical check for some x86 architectural msrs
Several architectural msrs (e.g MSR_KERNEL_GS_BASE) must contain
a canonical address, and according to Intel PRM, this is enforced
by a #GP canonical check during MSR write.
However as it turns out, the supported address width
used for this canonical check is determined only
by host cpu model:
if CPU *supports* 5 level paging, the width will be 57
regardless of the state of CR4.LA57.
Experemental tests on a Sapphire Rapids CPU and on a Zen4 CPU
confirm this behavior.
In addition to that, the Intel ISA extension manual mentions that this might
be the architectural behavior:
Architecture Instruction Set Extensions and Future Features Programming Reference [1].
Chapter 6.4:
"CANONICALITY CHECKING FOR DATA ADDRESSES WRITTEN TO CONTROL REGISTERS AND
MSRS"
"In Processors that support LAM continue to require the addresses written to
control registers or MSRs to be 57-bit canonical if the processor _supports_
5-level paging or 48-bit canonical if it supports only 4-level paging"
[1]: https://cdrdv2.intel.com/v1/dl/getContent/671368
Suggested-by: Chao Gao <chao.gao@...el.com>
Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
---
arch/x86/kvm/x86.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a6968eadd418..3582f0bb7644 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1844,7 +1844,16 @@ static int __kvm_set_msr(struct kvm_vcpu *vcpu, u32 index, u64 data,
case MSR_KERNEL_GS_BASE:
case MSR_CSTAR:
case MSR_LSTAR:
- if (is_noncanonical_address(data, vcpu))
+
+ /*
+ * Both AMD and Intel cpus allow values which
+ * are canonical in the 5 level paging mode but are not
+ * canonical in the 4 level paging mode to be written
+ * to the above MSRs, as long as the host CPU supports
+ * 5 level paging, regardless of the state of the CR4.LA57.
+ */
+ if (!__is_canonical_address(data,
+ kvm_cpu_cap_has(X86_FEATURE_LA57) ? 57 : 48))
return 1;
break;
case MSR_IA32_SYSENTER_EIP:
--
2.40.1
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