Re: [PATCH 6.1.y 1/4] KVM: x86/pmu: Gate all "unimplemented MSR" prints on report_ignored_msrs

[Sean Christopherson <seanjc@google.com>]

On Thu, Aug 14, 2025, Sasha Levin wrote:
> From: Sean Christopherson <seanjc@google.com>
> 
> [ Upstream commit e76ae52747a82a548742107b4100e90da41a624d ]
> 
> Add helpers to print unimplemented MSR accesses and condition all such
> prints on report_ignored_msrs, i.e. honor userspace's request to not
> print unimplemented MSRs.  Even though vcpu_unimpl() is ratelimited,
> printing can still be problematic, e.g. if a print gets stalled when host
> userspace is writing MSRs during live migration, an effective stall can
> result in very noticeable disruption in the guest.
> 
> E.g. the profile below was taken while calling KVM_SET_MSRS on the PMU
> counters while the PMU was disabled in KVM.
> 
>   -   99.75%     0.00%  [.] __ioctl
>    - __ioctl
>       - 99.74% entry_SYSCALL_64_after_hwframe
>            do_syscall_64
>            sys_ioctl
>          - do_vfs_ioctl
>             - 92.48% kvm_vcpu_ioctl
>                - kvm_arch_vcpu_ioctl
>                   - 85.12% kvm_set_msr_ignored_check
>                        svm_set_msr
>                        kvm_set_msr_common
>                        printk
>                        vprintk_func
>                        vprintk_default
>                        vprintk_emit
>                        console_unlock
>                        call_console_drivers
>                        univ8250_console_write
>                        serial8250_console_write
>                        uart_console_write
> 
> Reported-by: Aaron Lewis <aaronlewis@google.com>
> Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> Link: https://lore.kernel.org/r/20230124234905.3774678-3-seanjc@google.com
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> Stable-dep-of: 7d0cce6cbe71 ("KVM: VMX: Wrap all accesses to IA32_DEBUGCTL with getter/setter APIs")
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---

Acked-by: Sean Christopherson <seanjc@google.com>