linux-kernel - Re: [PATCH v9 18/19] KVM: selftests: Add ucall support for TDX

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Message-ID: <aLcRIn8ryB2kXWcD@google.com>
Date: Tue, 2 Sep 2025 08:45:38 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Binbin Wu <binbin.wu@...ux.intel.com>
Cc: Sagi Shahar <sagis@...gle.com>, linux-kselftest@...r.kernel.org, 
	Paolo Bonzini <pbonzini@...hat.com>, Shuah Khan <shuah@...nel.org>, 
	Ackerley Tng <ackerleytng@...gle.com>, Ryan Afranji <afranji@...gle.com>, 
	Andrew Jones <ajones@...tanamicro.com>, Isaku Yamahata <isaku.yamahata@...el.com>, 
	Erdem Aktas <erdemaktas@...gle.com>, Rick Edgecombe <rick.p.edgecombe@...el.com>, 
	Roger Wang <runanwang@...gle.com>, Oliver Upton <oliver.upton@...ux.dev>, 
	"Pratik R. Sampat" <pratikrajesh.sampat@....com>, Reinette Chatre <reinette.chatre@...el.com>, 
	Ira Weiny <ira.weiny@...el.com>, Chao Gao <chao.gao@...el.com>, 
	Chenyi Qiang <chenyi.qiang@...el.com>, linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v9 18/19] KVM: selftests: Add ucall support for TDX

On Wed, Aug 27, 2025, Binbin Wu wrote:
> On 8/21/2025 12:29 PM, Sagi Shahar wrote:
> > @@ -46,11 +69,23 @@ void *ucall_arch_get_ucall(struct kvm_vcpu *vcpu)
> >   {
> >   	struct kvm_run *run = vcpu->run;
> > -	if (run->exit_reason == KVM_EXIT_IO && run->io.port == UCALL_PIO_PORT) {
> > -		struct kvm_regs regs;
> > +	switch (vm_type) {
> > +	case KVM_X86_TDX_VM:
> > +		if (vcpu->run->exit_reason == KVM_EXIT_MMIO &&
> > +		    vcpu->run->mmio.phys_addr == host_ucall_mmio_gpa &&
> > +		    vcpu->run->mmio.len == 8 && vcpu->run->mmio.is_write) {
> > +			uint64_t data = *(uint64_t *)vcpu->run->mmio.data;
> > +
> > +			return (void *)data;
> > +		}
> > +		return NULL;
> 
> My first thought was how did SEV_ES or SNP work for this since they are not
> able to get RDI neither.
> Then I had a check in sev_smoke_test.c, both guest_sev_es_code() and
> guest_snp_code() call GUEST_ASSERT(), which finally calls ucall_assert(), but
> in test_sev(), the code doesn't handle ucall for SEV_ES or SNP.
> Does it mean GUEST_ASSERT() is currently not working and ignored for SEV_ES
> and SNP? Or did I miss anything?

GUEST_ASSERT() "works" for -ES and -SNP in the sense that it generates as test
failure due to the #VC not being handled (leads to SHUTDOWN).  But you're correct
that ucall isn't functional yet.  x86/sev_smoke_test.c fudges around lack of ucall
by using the GHCB MSR protocol to signal "done".

        /*
         * TODO: Add GHCB and ucall support for SEV-ES guests.  For now, simply
         * force "termination" to signal "done" via the GHCB MSR protocol.
         */
        wrmsr(MSR_AMD64_SEV_ES_GHCB, GHCB_MSR_TERM_REQ);
        vmgexit();