6.6-stable review patch. If anyone has any objections, please let me know.
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From: Sean Christopherson seanjc@google.com
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 ravi.bangoria@amd.com Cc: stable@vger.kernel.org Reviewed-and-tested-by: Ravi Bangoria ravi.bangoria@amd.com Link: https://lore.kernel.org/r/20250227222411.3490595-3-seanjc@google.com Signed-off-by: Sean Christopherson seanjc@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/kvm/svm/svm.c | 9 +++++++++ arch/x86/kvm/svm/svm.h | 2 +- 2 files changed, 10 insertions(+), 1 deletion(-)
--- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -3168,6 +3168,15 @@ static int svm_set_msr(struct kvm_vcpu * */ 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;
--- a/arch/x86/kvm/svm/svm.h +++ b/arch/x86/kvm/svm/svm.h @@ -533,7 +533,7 @@ static inline bool is_vnmi_enabled(struc /* 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;