Re: [RFC PATCH 00/35] SEV-ES hypervisor support

From: Sean Christopherson <seanjc@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	x86@kernel.org, Jim Mattson <jmattson@google.com>,
	Joerg Roedel <joro@8bytes.org>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Borislav Petkov <bp@alien8.de>, Ingo Molnar <mingo@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Brijesh Singh <brijesh.singh@amd.com>,
	Xiaoyao Li <xiaoyao.li@intel.com>,
	Isaku Yamahata <isaku.yamahata@intel.com>
Subject: Re: [RFC PATCH 00/35] SEV-ES hypervisor support
Date: Mon, 30 Nov 2020 19:35:19 +0000	[thread overview]
Message-ID: <X8VJdxTKKkC7uEMh@google.com> (raw)
In-Reply-To: <8759948d-aa0b-3929-bda6-488b6788489a@redhat.com>

+Isaku and Xiaoyao

On Mon, Nov 30, 2020, Paolo Bonzini wrote:
> On 30/11/20 19:14, Sean Christopherson wrote:
> > > > TDX also selectively blocks/skips portions of other ioctl()s so that the
> > > > TDX code itself can yell loudly if e.g. .get_cpl() is invoked.  The event
> > > > injection restrictions are due to direct injection not being allowed (except
> > > > for NMIs); all IRQs have to be routed through APICv (posted interrupts) and
> > > > exception injection is completely disallowed.
> > > > 
> > > >     kvm_vcpu_ioctl_x86_get_vcpu_events:
> > > > 	if (!vcpu->kvm->arch.guest_state_protected)
> > > >           	events->interrupt.shadow = kvm_x86_ops.get_interrupt_shadow(vcpu);
> > > Perhaps an alternative implementation can enter the vCPU with immediate exit
> > > until no events are pending, and then return all zeroes?
> > 
> > This can't work.  If the guest has STI blocking, e.g. it did STI->TDVMCALL with
> > a valid vIRQ in GUEST_RVI, then events->interrupt.shadow should technically be
> > non-zero to reflect the STI blocking.  But, the immediate exit (a hardware IRQ
> > for TDX guests) will cause VM-Exit before the guest can execute any instructions
> > and thus the guest will never clear STI blocking and never consume the pending
> > event.  Or there could be a valid vIRQ, but GUEST_RFLAGS.IF=0, in which case KVM
> > would need to run the guest for an indeterminate amount of time to wait for the
> > vIRQ to be consumed.
> 
> Delayed interrupts are fine, since they are injected according to RVI and
> the posted interrupt descriptor.  I'm thinking more of events (exceptions
> and interrupts) that caused an EPT violation exit and were recorded in the
> IDT-vectored info field.

Ah.  As is, I don't believe KVM has access to this information.  TDX-Module
handles the actual EPT violation, as well as event reinjection.  The EPT
violation reported by SEAMRET is synthesized, and IIRC the IDT-vectoring field
is not readable.

Regardless, is there an actual a problem with having a "pending" exception that
isn't reported to userspace?  Obviously the info needs to be migrated, but that
will be taken care of by virtue of migrating the VMCS.