On Mon, May 15, 2023 at 02:53:07PM +0800, Xiaoyao Li wrote: >On 4/14/2023 2:25 PM, Chao Gao wrote: >> From: Zhang Chen <chen.zhang@xxxxxxxxx> >> >> Add a kvm-only CPUID feature leaf for CPUID.7.2.EDX and RRSBA_CTRL >> as the first feature in the leaf. >> >> RRSBA_CTRL is enumerated by CPUID.7.2.EDX[2]. If supported, RRSBA_DIS_U >> (bit 5) and RRSBA_DIS_S (bit 6) of IA32_SPEC_CTRL MSR can be used to >> disable RRSBA behavior for CPL3 and CPL0/1/2 respectively. >> >> Note that KVM does not intercept guests' IA32_SPEC_CTRL MSR accesses >> after a non-zero is written to the MSR. Therefore, guests can already >> toggle the two bits if the host supports RRSBA_CTRL, and no extra code >> is needed to allow guests to toggle the two bits. > >This is a bug that also matters with other bits in MSR_IA32_SPEC_CTRL which >has a dedicated enumeration CPUID bit and no support in KVM yet. Do you mean passing through the MSR is a bug? guest can write any hardware supported value to the MSR if the MSR isn't intercepted. I guess this is intentional and a trade-off for performance (note that context-switch may cause writes to the MSR). And see commit 841c2be09fe4 ("kvm: x86: replace kvm_spec_ctrl_test_value with runtime test on the host") it appears that this behavior is widely recognized. > >I think we need to fix this bug at first. I have no idea how to fix the "bug" without intercepting the MSR. The performance penalty makes me think intercepting the MSR is not a viable solution. > >> Signed-off-by: Zhang Chen <chen.zhang@xxxxxxxxx> >> Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx> >> Tested-by: Jiaan Lu <jiaan.lu@xxxxxxxxx> >> --- >> arch/x86/kvm/cpuid.c | 22 +++++++++++++++++++--- >> arch/x86/kvm/reverse_cpuid.h | 7 +++++++ >> 2 files changed, 26 insertions(+), 3 deletions(-) >> >> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c >> index 9583a110cf5f..f024c3ac2203 100644 >> --- a/arch/x86/kvm/cpuid.c >> +++ b/arch/x86/kvm/cpuid.c >> @@ -685,6 +685,10 @@ void kvm_set_cpu_caps(void) >> SF(SGX1) | SF(SGX2) | SF(SGX_EDECCSSA) >> ); >> + kvm_cpu_cap_init_kvm_defined(CPUID_7_2_EDX, >> + SF(RRSBA_CTRL) >> + ); >> + > >Please move this hook up to right follow the leaf CPUID_7_1_EAX. sure. will do.