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

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Message-ID: <X8U2gyj7F2wFU3JI@google.com>
Date:   Mon, 30 Nov 2020 18:14:27 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Tom Lendacky <thomas.lendacky@....com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [RFC PATCH 00/35] SEV-ES hypervisor support

On Mon, Nov 30, 2020, Paolo Bonzini wrote:
> On 16/09/20 02:19, 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.

Tangentially related, I haven't looked through the official external TDX docs,
but I suspect that vmcs.GUEST_RVI is listed as inaccessible for production TDs.
This will be changed as the VMM needs access to GUEST_RVI to handle
STI->TDVMCALL(HLT), otherwise the VMM may incorrectly put the vCPU into a
blocked (not runnable) state even though it has a pending wake event.