Messages in this thread | | | Date | Thu, 11 Apr 2024 09:49:55 +0200 | Subject | Re: [PATCH] KVM: x86: Set BHI_NO in guest when host is not affected by BHI | From | Alexandre Chartre <> |
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On 4/11/24 09:34, Nikolay Borisov wrote: > > > On 11.04.24 г. 10:24 ч., Alexandre Chartre wrote: >> When a system is not affected by the BHI bug then KVM should >> configure guests with BHI_NO to ensure they won't enable any >> BHI mitigation. >> >> Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com> >> --- >> arch/x86/kvm/x86.c | 3 +++ >> 1 file changed, 3 insertions(+) >> >> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c >> index 984ea2089efc..f43d3c15a6b7 100644 >> --- a/arch/x86/kvm/x86.c >> +++ b/arch/x86/kvm/x86.c >> @@ -1678,6 +1678,9 @@ static u64 kvm_get_arch_capabilities(void) >> if (!boot_cpu_has_bug(X86_BUG_GDS) || gds_ucode_mitigated()) >> data |= ARCH_CAP_GDS_NO; >> + if (!boot_cpu_has_bug(X86_BUG_BHI)) >> + data |= ARCH_CAP_BHI_NO; >> + > > But this is already handled since ARCH_CAP_BHI_NO is added to > KVM_SUPPORTED_ARCH_CAP so when the host caps are read that bit is > going to be set there, if it's set for the physical cpu of course.
Correct, if the host has ARCH_CAP_BHI_NO then it will be propagated to the guest. But the host might not have ARCH_CAP_BHI_NO set and not be affected by BHI.
That's the case for example of Skylake servers: they don't have ARCH_CAP_BHI_NO, but they are not affected by BHI because they don't have eIBRS. However, a guest will think it is affected because it doesn't know if eIBRS is present on the system (because virtualization can hide it).
I tested on Skylake:
Without the patch, both host and guest are running 6.9.0-rc3, then BHI mitigations are:
- Host: BHI: Not affected - Guest: BHI: SW loop, KVM: SW loop
=> so guest enables BHI SW loop mitigation although host doesn't need mitigation.
With the patch on the host, guest still running 6.9.0-rc3, then BHI mitigations are:
- Host: BHI: Not affected - Guest: BHI: Not affected
=> now guest doesn't enable BHI mitigation, like the host.
Thanks,
alex.
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