Re: [PATCH V2 2/2] x86/tdx: Skip clearing reclaimed pages unless X86_BUG_TDX_PW_MCE is present

[Vishal Annapurve]

On Thu, Jul 3, 2025 at 8:37 AM Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
>
> Avoid clearing reclaimed TDX private pages unless the platform is affected
> by the X86_BUG_TDX_PW_MCE erratum. This significantly reduces VM shutdown
> time on unaffected systems.
>
> Background
>
> KVM currently clears reclaimed TDX private pages using MOVDIR64B, which:
>
>    - Clears the TD Owner bit (which identifies TDX private memory) and
>      integrity metadata without triggering integrity violations.
>    - Clears poison from cache lines without consuming it, avoiding MCEs on
>      access (refer TDX Module Base spec. 16.5. Handling Machine Check
>      Events during Guest TD Operation).
>
> The TDX module also uses MOVDIR64B to initialize private pages before use.
> If cache flushing is needed, it sets TDX_FEATURES.CLFLUSH_BEFORE_ALLOC.
> However, KVM currently flushes unconditionally, refer commit 94c477a751c7b
> ("x86/virt/tdx: Add SEAMCALL wrappers to add TD private pages")
>
> In contrast, when private pages are reclaimed, the TDX Module handles
> flushing via the TDH.PHYMEM.CACHE.WB SEAMCALL.
>
> Problem
>
> Clearing all private pages during VM shutdown is costly. For guests
> with a large amount of memory it can take minutes.
>
> Solution
>
> TDX Module Base Architecture spec. documents that private pages reclaimed
> from a TD should be initialized using MOVDIR64B, in order to avoid
> integrity violation or TD bit mismatch detection when later being read
> using a shared HKID, refer April 2025 spec. "Page Initialization" in
> section "8.6.2. Platforms not Using ACT: Required Cache Flush and
> Initialization by the Host VMM"
>
> That is an overstatement and will be clarified in coming versions of the
> spec. In fact, as outlined in "Table 16.2: Non-ACT Platforms Checks on
> Memory" and "Table 16.3: Non-ACT Platforms Checks on Memory Reads in Li
> Mode" in the same spec, there is no issue accessing such reclaimed pages
> using a shared key that does not have integrity enabled. Linux always uses
> KeyID 0 which never has integrity enabled. KeyID 0 is also the TME KeyID
> which disallows integrity, refer "TME Policy/Encryption Algorithm" bit
> description in "Intel Architecture Memory Encryption Technologies" spec
> version 1.6 April 2025. So there is no need to clear pages to avoid
> integrity violations.
>
> There remains a risk of poison consumption. However, in the context of
> TDX, it is expected that there would be a machine check associated with the
> original poisoning. On some platforms that results in a panic. However
> platforms may support "SEAM_NR" Machine Check capability, in which case
> Linux machine check handler marks the page as poisoned, which prevents it
> from being allocated anymore, refer commit 7911f145de5fe ("x86/mce:
> Implement recovery for errors in TDX/SEAM non-root mode")
>
> Improvement
>
> By skipping the clearing step on unaffected platforms, shutdown time
> can improve by up to 40%.

This patch looks good to me.

I would like to raise a related topic, is there any requirement for
zeroing pages on conversion from private to shared before
userspace/guest faults in the gpa ranges as shared?

If the answer is no for all CoCo architectures then guest_memfd can
simply just zero pages on allocation for all it's users and not worry
about zeroing later.