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

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Message-ID: <c7b8f395-579b-40d9-b3eb-29a347f73ec9@intel.com>
Date: Mon, 4 Nov 2024 09:56:58 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Adrian Hunter <adrian.hunter@...el.com>
Subject: Re: [PATCH 1/2] KVM: VMX: Bury Intel PT virtualization (guest/host
 mode) behind CONFIG_BROKEN

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@...r.kernel.org
> Cc: Adrian Hunter <adrian.hunter@...el.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.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;
>