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

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 11/5/2024 6:46 AM, Sean Christopherson wrote:
> On Mon, Nov 04, 2024, Xiaoyao Li wrote:
>> 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.
> 
> Ya, BROKEN is typically used directly in Kconfigs, e.g. "depends on BROKEN".  But
> I can't think of any reason using it in this way would be problematic.

I see. Thanks for the information!

Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>