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

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Date:   Mon, 30 Nov 2020 19:35:01 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.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 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.

Paolo

> 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.
>