On Mon, May 22, 2023 at 01:54:55PM -0700, Pawan Gupta wrote:
> On Mon, May 22, 2023 at 10:43:49AM -0700, Sean Christopherson wrote:
> > On Fri, May 19, 2023, Pawan Gupta wrote:
> > > On Thu, May 18, 2023 at 10:33:15AM -0700, Sean Christopherson wrote:
> > > > I made the mistake of digging into why KVM doesn't advertise ARCH_CAP_FB_CLEAR_CTRL...
> > > >
> > > > 1. I see *nothing* in commit 027bbb884be0 ("KVM: x86/speculation: Disable Fill
> > > > buffer clear within guests") that justifies 1x RDMSR and 2x WRMSR on every
> > > > entry+exit.
> > >
> > > Unnecessary VERWs in guest will have much higher impact than due to MSR
> > > read/write at vmentry/exit.
> >
> > Can you provide numbers for something closeish to a real world workload?
>
> I am collecting the numbers, will update here soon.
Looks like avoiding VERW flush behavior in guest results in 2-5%
improvement in performance.
Pybench and CPU bound sysbench test shows minor improvement when
avoiding reading/writing MSR_MCU_OPT_CTRL at VMentry/VMexit.
Baseline : v6.4-rc3
Target : v6.4-rc3 + No read/write to MSR_MCU_OPT_CTRL at VMentry/VMexit
| Test | Configuration | Relative |
| --------- | ------------- | -------- |
| nginx | 200 | 0.977 |
| hackbench | 8 - Process | 0.975 |
| sysbench | RAM / Memory | 0.946 |
| sysbench | CPU | 1.002 |
| pybench | T.F.A.T.T | 1.007 |
Host configuration (Icelake server):
CPU family: 6
Model: 106
Stepping: 6
Vulnerability Itlb multihit: Not affected
Vulnerability L1tf: Not affected
Vulnerability Mds: Not affected
Vulnerability Meltdown: Not affected
Vulnerability Mmio stale data: Mitigation; Clear CPU buffers; SMT vulnerable
Vulnerability Retbleed: Not affected
Vulnerability Spec store bypass: Mitigation; Speculative Store Bypass disabled via prctl
Vulnerability Spectre v1: Mitigation; usercopy/swapgs barriers and __user pointer sanitization
Vulnerability Spectre v2: Vulnerable: eIBRS with unprivileged eBPF
Vulnerability Srbds: Not affected
Vulnerability Tsx async abort: Not affected
Guest configuration (Skylake Client):
CPU family: 6
Model: 94
Stepping: 3
Vulnerabilities:
Itlb multihit: KVM: Mitigation: VMX unsupported
L1tf: Mitigation; PTE Inversion
Mds: Vulnerable: Clear CPU buffers attempted, no microcode;
SMT Host state unknown
Meltdown: Mitigation; PTI
Mmio stale data: Vulnerable: Clear CPU buffers attempted, no microcode;
SMT Host state unknown
Retbleed: Vulnerable
Spec store bypass: Vulnerable
Spectre v1: Mitigation; usercopy/swapgs barriers and __user pointer
sanitization
Spectre v2: Mitigation; Retpolines, STIBP disabled, RSB filling, PB
RSB-eIBRS Not affected
Srbds: Unknown: Dependent on hypervisor status
Tsx async abort: Vulnerable: Clear CPU buffers attempted, no microcode;
SMT Host state unknown
