Messages in this thread |  | | From | "Liu, Jing2" <> | Subject | Re: [PATCH RFC 2/7] kvm: x86: Introduce XFD MSRs as passthrough to guest | Date | Wed, 2 Jun 2021 11:12:57 +0800 |
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On 5/25/2021 5:43 AM, Sean Christopherson wrote: > On Sun, Feb 07, 2021, Jing Liu wrote: >> Passthrough both MSRs to let guest access and write without vmexit. > Why? Except for read-only MSRs, e.g. MSR_CORE_C1_RES, passthrough MSRs are > costly to support because KVM must context switch the MSR (which, by the by, is > completely missing from the patch). > > In other words, if these MSRs are full RW passthrough, guests with XFD enabled > will need to load the guest value on entry, save the guest value on exit, and > load the host value on exit. That's in the neighborhood of a 40% increase in > latency for a single VM-Enter/VM-Exit roundtrip (~1500 cycles => >2000 cycles). > > I'm not saying these can't be passhthrough, but there needs to be strong > justification for letting the guest read/write them directly. For IA32_XFD, it's per task and switched during task switch(if different). Meanwhile, hardware uses IA32_XFD to prevent any instruction per task touching AMX state at the first time and generate #NM. This means if vcpu running with AMX enabled, hardware IA32_XFD should keep a guest value, and once guest #NM handler finished, the bit should not be set anymore for this task. No matter passthrough or not, IA32_XFD need be restored to guest value when vm-enter, and load host value on exit (or load zero to prevent losing guest AMX in use state). And passthrough makes guest IA32_XFD switch during task switch without trapped.
For IA32_XFD_ERR, hardware automatically set the bit once #NM fault. #NM handler detect and clear the bit. No matter passthrough or not, KVM doesn't know if guest has #NM, it need read and save IA32_XFD_ERR when vmexit to prevent preemption task from clearing bit. Emulating need not restore guest non-zero IA32_XFD_ERR when vmenter and effort is guest #NM handler takes extra trap(s). But restoring a non-zero IA32_XFD_ERR only occurs for the vmexit case: between task's first AMX instruction(causing #NM) and end of clearing in guest #NM handler.
Thanks, Jing
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