Re: [PATCH 1/2] KVM: VMX: Bury Intel PT virtualization (guest/host mode) behind CONFIG_BROKEN

Xiaoyao Li
2024-11-04T01:57:13Z

On 11/2/2024 2:50 AM, Sean Christopherson wrote:
> Hide KVM's pt_mode module param behind CONFIG_BROKEN, i.e. disable support
> for virtualizing Intel PT via guest/host mode unless BROKEN=y. There are
> myriad bugs in the implementation, some of which are fatal to the guest,
> and others which put the stability and health of the host at risk.
>
> For guest fatalities, the most glaring issue is that KVM fails to ensure
> tracing is disabled, and *stays* disabled prior to VM-Enter, which is
> necessary as hardware disallows loading (the guest's) RTIT_CTL if tracing
> is enabled (enforced via a VMX consistency check). Per the SDM:
>
> If the logical processor is operating with Intel PT enabled (if
> IA32_RTIT_CTL.TraceEn = 1) at the time of VM entry, the "load
> IA32_RTIT_CTL" VM-entry control must be 0.
>
> On the host side, KVM doesn't validate the guest CPUID configuration
> provided by userspace, and even worse, uses the guest configuration to
> decide what MSRs to save/load at VM-Enter and VM-Exit. E.g. configuring
> guest CPUID to enumerate more address ranges than are supported in hardware
> will result in KVM trying to passthrough, save, and load non-existent MSRs,
> which generates a variety of WARNs, ToPA ERRORs in the host, a potential
> deadlock, etc.
>
> Fixes: f99e3daf94ff ("KVM: x86: Add Intel PT virtualization work mode")
> Cc: stable@vger.kernel.org
> Cc: Adrian Hunter <adrian.hunter@intel.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> arch/x86/kvm/vmx/vmx.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 6ed801ffe33f..087504fb1589 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -217,9 +217,11 @@ module_param(ple_window_shrink, uint, 0444);
> static unsigned int ple_window_max = KVM_VMX_DEFAULT_PLE_WINDOW_MAX;
> module_param(ple_window_max, uint, 0444);
>
> -/* Default is SYSTEM mode, 1 for host-guest mode */
> +/* Default is SYSTEM mode, 1 for host-guest mode (which is BROKEN) */
> int __read_mostly pt_mode = PT_MODE_SYSTEM;
> +#ifdef CONFIG_BROKEN
> module_param(pt_mode, int, S_IRUGO);
> +#endif

I like the patch, but I didn't find any other usercase of CONFIG_BROKEN
in current Linux.

> struct x86_pmu_lbr __ro_after_init vmx_lbr_caps;
>