Re: [PATCH 3/4] x86/virt/tdx: Add SEAMCALL wrapper for TDH.SYS.DISABLE

[Edgecombe, Rick P]

On Mon, 2026-03-16 at 11:51 +0000, Kiryl Shutsemau wrote:
> On Fri, Mar 06, 2026 at 05:03:57PM -0800, Rick Edgecombe wrote:
> > From: Vishal Verma <vishal.l.verma@xxxxxxxxx>
> > 
> > Some early TDX-capable platforms have an erratum where a partial write
> > to TDX private memory can cause a machine check on a subsequent read.
> > On these platforms, kexec and kdump have been disabled in these cases,
> > because the old kernel cannot safely hand off TDX state to the new
> > kernel. Later TDX modules support the TDH.SYS.DISABLE SEAMCALL, which
> > provides a way to cleanly disable TDX and allow kexec to proceed.
> 
> Does it need to be enumerated?
> 
> I don't see this SEAMCALL be covered in the public documentation.
> </me looking around>
> Ah! Found it the the draft. So the feature is not yet finalized.
> 
> "Support of TDH.SYS.DISABLE is enumerated by TDX_FEATURES0. SYS_DISABLE
> (bit 53)"
> 
> I am seeing the next patch calling it unconditionally. Is it okay?

We debated checking the feature bit before allowing kexec, but decided it was
simpler to just blindly call and ignore the errors. The reasoning was that this
is already a somewhat exotic scenario being addressed, and future modules will
have the feature. So maintaining a check for the feature bit only helps a little
bit, for a short time. And then only if the user would rather have kexec blocked
than attempt it. Do you think it is worth it?

> 
> > This can be a long running operation, and the time needed largely
> > depends on the amount of memory that has been allocated to TDs. If all
> > TDs have been destroyed prior to the sys_disable call, then it is fast,
> > with only needing to override the TDX module memory.
> > 
> > After the SEAMCALL completes, the TDX module is disabled and all memory
> > resources allocated to TDX are freed and reset. The next kernel can then
> > re-initialize the TDX module from scratch via the normal TDX bring-up
> > sequence.
> > 
> > The SEAMCALL may be interrupted by an interrupt. In this case, it
> > returns TDX_INTERRUPTED_RESUMABLE, and it must be retried in a loop
> > until the operation completes successfully.
> > 
> > Add a tdx_sys_disable() helper, which implements the retry loop around
> > the SEAMCALL to provide this functionality.
> > 
> > Signed-off-by: Vishal Verma <vishal.l.verma@xxxxxxxxx>
> > Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> > ---
> >   arch/x86/include/asm/tdx.h  |  3 +++
> >   arch/x86/virt/vmx/tdx/tdx.c | 18 ++++++++++++++++++
> >   arch/x86/virt/vmx/tdx/tdx.h |  1 +
> >   3 files changed, 22 insertions(+)
> > 
> > diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
> > index f0826b0a512a..baaf43a09e99 100644
> > --- a/arch/x86/include/asm/tdx.h
> > +++ b/arch/x86/include/asm/tdx.h
> > @@ -173,6 +173,8 @@ static inline int pg_level_to_tdx_sept_level(enum pg_level level)
> >           return level - 1;
> >   }
> >   
> > +void tdx_sys_disable(void);
> > +
> >   u64 tdh_vp_enter(struct tdx_vp *vp, struct tdx_module_args *args);
> >   u64 tdh_mng_addcx(struct tdx_td *td, struct page *tdcs_page);
> >   u64 tdh_mem_page_add(struct tdx_td *td, u64 gpa, struct page *page, struct page *source, u64 *ext_err1, u64 *ext_err2);
> > @@ -204,6 +206,7 @@ static inline void tdx_init(void) { }
> >   static inline u32 tdx_get_nr_guest_keyids(void) { return 0; }
> >   static inline const char *tdx_dump_mce_info(struct mce *m) { return NULL; }
> >   static inline const struct tdx_sys_info *tdx_get_sysinfo(void) { return NULL; }
> > +static inline void tdx_sys_disable(void) { }
> >   #endif	/* CONFIG_INTEL_TDX_HOST */
> >   
> >   #endif /* !__ASSEMBLER__ */
> > diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
> > index 0802d0fd18a4..68bd2618dde4 100644
> > --- a/arch/x86/virt/vmx/tdx/tdx.c
> > +++ b/arch/x86/virt/vmx/tdx/tdx.c
> > @@ -37,6 +37,7 @@
> >   #include <asm/msr.h>
> >   #include <asm/cpufeature.h>
> >   #include <asm/tdx.h>
> > +#include <asm/shared/tdx_errno.h>
> >   #include <asm/cpu_device_id.h>
> >   #include <asm/processor.h>
> >   #include <asm/mce.h>
> > @@ -1940,3 +1941,20 @@ u64 tdh_phymem_page_wbinvd_hkid(u64 hkid, struct page *page)
> >   	return seamcall(TDH_PHYMEM_PAGE_WBINVD, &args);
> >   }
> >   EXPORT_SYMBOL_FOR_KVM(tdh_phymem_page_wbinvd_hkid);
> > +
> > +void tdx_sys_disable(void)
> > +{
> > +	struct tdx_module_args args = {};
> > +
> > +	/*
> > +	 * SEAMCALLs that can return TDX_INTERRUPTED_RESUMABLE are guaranteed
> > +	 * to make forward progress between interrupts, so it is safe to loop
> > +	 * unconditionally here.
> > +	 *
> > +	 * This is a 'destructive' SEAMCALL, in that no other SEAMCALL can be
> > +	 * run after this until a full reinitialization is done.
> > +	 */
> > +	while (seamcall(TDH_SYS_DISABLE, &args) == TDX_INTERRUPTED_RESUMABLE)
> > +		;
> 
> Silently ignore any other errors?

Do you think it's worth a warn? There are a couple other considerations.
  - Kai brought up offline that we should handle TDX_SYS_BUSY here too.
  - Previous kexec patches had trouble solving races around tdx enabling. So we
have to handle the seamcall failures.

So we have to exclude a few different errors in different ways. And then the
warn worthy error codes either don't impact anything, or the new kernel will
fail to initialize the TDX module and give notice there.

I don't have a strong objection. It seems to be a judgment call of whether the
complexity is worth the benefit.