Re: [PATCH v16.1] KVM: selftests: Add support for TDX ucalls, via TDVMCALL_REPORT_FATAL_ERROR

[Binbin Wu <binbin.wu@linux.intel.com>]

On 11/7/2026 12:47 AM, Sean Christopherson wrote:
> Add support for doing ucalls on TDX by abusing TDVMCALL_REPORT_FATAL_ERROR
> to pass the address of the payload to the host.  The "fatal error" TDVMCALL
> is perfectly suited for passing information to host userspace, as both the
> TDX Module and KVM allow the guest to pass (almost) all registers to the
> host, i.e. provide enough of a data payload to make a collision with a real
> fatal error practically impossible.
> 
> TDX can't use port I/O, as the TDX ABI doesn't allow the guest to share
> arbitrary register state with the host on a port I/O exit, and the port I/O
> data payload is limited to 4 bytes, i.e. would potentially truncate the
> ucall address.
> 
> Alternatively, TDX could use MMIO, but using a magic emulated MMIO address
> is fragile (see the TODO in __vm_create()), especially for TDX since TDX
> doesn't support read-only memslots, i.e. doesn't have line of sight towards
> addressing the TODO.  E.g. TDX could hardcode the address to something that
> is all but guaranteed to be unused on x86, e.g. the I/O APIC base address
> or the HPET address, but that doesn't truly address the fragility concerns,
> and it's ugly because ucall_arch_init() would completely ignore the passed
> in @mmio_gpa despite obviously utilizing emulated MMIO.
> 
> Use a simple flag to route vCPUs/VMs to the right do_ucall() routine, and
> take care to set the flag via write_guest_global() so that ucall_arch_init()
> is thread safe (in the *extremely* unlikely scenario a single test creates
> both TDX and non-TDX VMs *concurrently).
                                       ^
Nit: missing a '*'?

> 
> Alternatively, a virtual function pointer could be used to dispatch to the
> correct do_ucall() via an indirect branch, but that has zero advantages
> other than making ucall_arch_do_ucall() look prettier, at the cost of being
> slightly harder to understand and making ucall_arch_init() uglier.  KISS.
> 
> Cc: Sagi Shahar <sagis@google.com>
> Cc: Lisa Wang <wyihan@google.com>
> Cc: Xiaoyao Li <xiaoyao.li@intel.com>
> Cc: Binbin Wu <binbin.wu@linux.intel.com>
> Cc: Ackerley Tng <ackerleytng@google.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>

BTW, the in-reply-to patch didn't trigger a new round of Sashiko review.