From 6622db50d800ec0091db3acee54c64b2f60e6962 Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Thu, 27 Feb 2025 14:24:07 -0800 Subject: [PATCH] KVM: SVM: Suppress DEBUGCTL.BTF on AMD commit d0eac42f5cecce009d315655bee341304fbe075e upstream. Mark BTF as reserved in DEBUGCTL on AMD, as KVM doesn't actually support BTF, and fully enabling BTF virtualization is non-trivial due to interactions with the emulator, guest_debug, #DB interception, nested SVM, etc. Don't inject #GP if the guest attempts to set BTF, as there's no way to communicate lack of support to the guest, and instead suppress the flag and treat the WRMSR as (partially) unsupported. In short, make KVM behave the same on AMD and Intel (VMX already squashes BTF). Note, due to other bugs in KVM's handling of DEBUGCTL, the only way BTF has "worked" in any capacity is if the guest simultaneously enables LBRs. Reported-by: Ravi Bangoria Cc: stable@vger.kernel.org Reviewed-and-tested-by: Ravi Bangoria Link: https://lore.kernel.org/r/20250227222411.3490595-3-seanjc@google.com Signed-off-by: Sean Christopherson Signed-off-by: Greg Kroah-Hartman --- arch/x86/kvm/svm/svm.c | 9 +++++++++ arch/x86/kvm/svm/svm.h | 2 +- 2 files changed, 10 insertions(+), 1 deletion(-) diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index e7c247ca687c2..e448ba5374b53 100644 --- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -3179,6 +3179,15 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr) */ data &= ~GENMASK(5, 2); + /* + * Suppress BTF as KVM doesn't virtualize BTF, but there's no + * way to communicate lack of support to the guest. + */ + if (data & DEBUGCTLMSR_BTF) { + kvm_pr_unimpl_wrmsr(vcpu, MSR_IA32_DEBUGCTLMSR, data); + data &= ~DEBUGCTLMSR_BTF; + } + if (data & DEBUGCTL_RESERVED_BITS) return 1; diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h index 8c1e24ca50b58..d114efac7af78 100644 --- a/arch/x86/kvm/svm/svm.h +++ b/arch/x86/kvm/svm/svm.h @@ -591,7 +591,7 @@ static inline bool is_vnmi_enabled(struct vcpu_svm *svm) /* svm.c */ #define MSR_INVALID 0xffffffffU -#define DEBUGCTL_RESERVED_BITS (~(DEBUGCTLMSR_BTF | DEBUGCTLMSR_LBR)) +#define DEBUGCTL_RESERVED_BITS (~DEBUGCTLMSR_LBR) extern bool dump_invalid_vmcb; -- 2.47.3