Messages in this thread | | | Subject | Re: [PATCH 3/8] kvm: vmx: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD down to the guest | From | Paolo Bonzini <> | Date | Tue, 9 Jan 2018 17:57:08 +0100 |
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On 09/01/2018 17:48, Liran Alon wrote: >>> >>> + if (have_spec_ctrl) { >>> + rdmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl); >>> + if (vmx->spec_ctrl != 0) >>> + wrmsrl(MSR_IA32_SPEC_CTRL, 0); > > As I said also on the AMD patch, I think this is a bug. > Intel specify that we should set IBRS bit even if it was already set on every #VMExit.
That's correct (though I'd like to understand _why_---I'm not inclined to blindly trust a spec), but for now it's saving a wrmsr of 0. That is quite obviously okay, and will be also okay after the bare-metal IBRS patches.
Of course the code will become something like
if (using_ibrs || vmx->spec_ctrl != 0) wrmsrl(MSR_IA32_SPEC_CTRL, host_ibrs);
optimizing the case where the host is using retpolines.
Paolo
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