Messages in this thread | | | Subject | Re: [PATCH] x86: vmx: Allow direct access to MSR_IA32_SPEC_CTRL | From | David Woodhouse <> | Date | Mon, 29 Jan 2018 10:37:44 +0000 |
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On Mon, 2018-01-29 at 10:43 +0100, KarimAllah Ahmed wrote: > On 01/29/2018 09:46 AM, David Woodhouse wrote: > > Reading the code and comparing with the SDM, I can't see where we're > > ever setting VM_EXIT_MSR_STORE_{ADDR,COUNT} except in the nested > > case... > Hmmm ... you are probably right! I think all users of this interface > always trap + update save area and never passthrough the MSR. That is > why only LOAD is needed *so far*. > > Okay, let me sort this out in v3 then.
I'm starting to think a variant of Ashok's patch might actually be the simpler approach, and not "premature optimisation". Especially if we need to support the !cpu_has_vmx_msr_bitmaps() case?
Start with vmx->spec_ctrl set to zero. When first touched, make it passthrough (but not atomically switched) and set a flag (e.g. "spec_ctrl_live") which triggers the 'restore_branch_speculation' and 'save_and_restrict_branch_speculation' behaviours. Except don't use those macros. Those can look something like
/* If this vCPU has touched SPEC_CTRL then restore its value if needed */ if (vmx->spec_ctrl_live && vmx->spec_ctrl) wrmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl); /* vmentry is serialising on affected CPUs, so the conditional branch is safe */
... and, respectively, ...
/* If this vCPU has touched SPEC_CTRL then save its value and ensure we have zero */ if (vmx->spec_ctrl_live) { rdmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl); if (vmx->spec_ctrl) wrmsrl(MSR_IA32_SPEC_CTRL, 0); }
Perhaps we can ditch the separate 'spec_ctrl_live' flag and check the pass-through MSR bitmap directly, in the case that it exists? [unhandled content-type:application/x-pkcs7-signature] | |