linux-kernel - Re: [PATCH RFC 1/2] KVM: TDX: Disable general support for MWAIT in guest

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Message-ID: <aKMzEYR4t4Btd7kC@google.com>
Date: Mon, 18 Aug 2025 07:05:57 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Adrian Hunter <adrian.hunter@...el.com>
Cc: pbonzini@...hat.com, kvm@...r.kernel.org, rick.p.edgecombe@...el.com, 
	kirill.shutemov@...ux.intel.com, kai.huang@...el.com, 
	reinette.chatre@...el.com, xiaoyao.li@...el.com, 
	tony.lindgren@...ux.intel.com, binbin.wu@...ux.intel.com, 
	isaku.yamahata@...el.com, linux-kernel@...r.kernel.org, yan.y.zhao@...el.com, 
	chao.gao@...el.com, ira.weiny@...el.com
Subject: Re: [PATCH RFC 1/2] KVM: TDX: Disable general support for MWAIT in guest

On Sat, Aug 16, 2025, Adrian Hunter wrote:
> TDX support for using the MWAIT instruction in a guest has issues, so
> disable it for now.
> 
> Background
> 
> Like VMX, TDX can allow the MWAIT instruction to be executed in a guest.
> Unlike VMX, TDX cannot necessarily provide for virtualization of MSRs that
> a guest might reasonably expect to exist as well.
> 
> For example, in the case of a Linux guest, the default idle driver
> intel_idle may access MSR_POWER_CTL or MSR_PKG_CST_CONFIG_CONTROL.  To
> virtualize those, KVM would need the guest not to enable #VE reduction,
> which is not something that KVM can control or even be aware of.  Note,
> however, that the consequent unchecked MSR access errors might be harmless.
> 
> Without #VE reduction enabled, the TDX Module will inject #VE for MSRs that
> it does not virtualize itself.  The guest can then hypercall the host VMM
> for a resolution.
> 
> With #VE reduction enabled, accessing MSRs such as the 2 above, results in
> the TDX Module injecting #GP.
> 
> Currently, Linux guest opts for #VE reduction unconditionally if it is
> available, refer reduce_unnecessary_ve().  However, the #VE reduction
> feature was not added to the TDX Module until versions 1.5.09 and 2.0.04.
> Refer https://github.com/intel/tdx-module/releases
> 
> There is also a further issue experienced by a Linux guest.  Prior to
> TDX Module versions 1.5.09 and 2.0.04, the Always-Running-APIC-Timer (ARAT)
> feature (CPUID leaf 6: EAX bit 2) is not exposed.  That results in cpuidle
> disabling the timer interrupt and invoking the Tick Broadcast framework
> to provide a wake-up.  Currently, that falls back to the PIT timer which
> does not work for TDX, resulting in the guest becoming stuck in the idle
> loop.
> 
> Conclusion
> 
> User's may expect TDX support of MWAIT in a guest to be similar to VMX
> support, but KVM cannot ensure that.  Consequently KVM should not expose
> the capability.
> 
> Fixes: 0186dd29a2518 ("KVM: TDX: add ioctl to initialize VM with TDX specific parameters")
> Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>
> ---

NAK.

Fix the guest, or wherever else in the pile there are issues.  KVM is NOT carrying
hack-a-fixes to workaround buggy software/firmware.  Been there, done that.