linux-kernel - Re: [PATCH v2 0/9] Move initializing SEV/SNP functionality to KVM

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Message-ID: <Z2MeN9z69ul3oGiN@google.com>
Date: Wed, 18 Dec 2024 11:10:47 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Ashish Kalra <ashish.kalra@....com>
Cc: Dionna Amalie Glaze <dionnaglaze@...gle.com>, pbonzini@...hat.com, tglx@...utronix.de, 
	mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org, 
	hpa@...or.com, thomas.lendacky@....com, john.allen@....com, 
	herbert@...dor.apana.org.au, davem@...emloft.net, michael.roth@....com, 
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-crypto@...r.kernel.org, linux-coco@...ts.linux.dev
Subject: Re: [PATCH v2 0/9] Move initializing SEV/SNP functionality to KVM

On Tue, Dec 17, 2024, Ashish Kalra wrote:
> On 12/17/2024 3:37 PM, Sean Christopherson wrote:
> > On Tue, Dec 17, 2024, Ashish Kalra wrote:
> >> On 12/17/2024 10:00 AM, Dionna Amalie Glaze wrote:
> >>> On Mon, Dec 16, 2024 at 3:57 PM Ashish Kalra <Ashish.Kalra@....com> wrote:
> >>>>
> >>>> From: Ashish Kalra <ashish.kalra@....com>
> >>>
> >>>> The on-demand SEV initialization support requires a fix in QEMU to
> >>>> remove check for SEV initialization to be done prior to launching
> >>>> SEV/SEV-ES VMs.
> >>>> NOTE: With the above fix for QEMU, older QEMU versions will be broken
> >>>> with respect to launching SEV/SEV-ES VMs with the newer kernel/KVM as
> >>>> older QEMU versions require SEV initialization to be done before
> >>>> launching SEV/SEV-ES VMs.
> >>>>
> >>>
> >>> I don't think this is okay. I think you need to introduce a KVM
> >>> capability to switch over to the new way of initializing SEV VMs and
> >>> deprecate the old way so it doesn't need to be supported for any new
> >>> additions to the interface.
> >>>
> >>
> >> But that means KVM will need to support both mechanisms of doing SEV
> >> initialization - during KVM module load time and the deferred/lazy
> >> (on-demand) SEV INIT during VM launch.
> > 
> > What's the QEMU change?  Dionna is right, we can't break userspace, but maybe
> > there's an alternative to supporting both models.
> 
> Here is the QEMU fix : (makes a SEV PLATFORM STATUS firmware call via PSP
> driver ioctl to check if SEV is in INIT state)
>  
> diff --git a/target/i386/sev.c b/target/i386/sev.c
> index 1a4eb1ada6..4fa8665395 100644
> --- a/target/i386/sev.c
> +++ b/target/i386/sev.c
> @@ -1503,15 +1503,6 @@ static int sev_common_kvm_init(ConfidentialGuestSupport *cgs, Error **errp)
>          }
>      }
> 
> -    if (sev_es_enabled() && !sev_snp_enabled()) {
> -        if (!(status.flags & SEV_STATUS_FLAGS_CONFIG_ES)) {
> -            error_setg(errp, "%s: guest policy requires SEV-ES, but "
> -                         "host SEV-ES support unavailable",
> -                         __func__);
> -            return -1;
> -        }
> -    }

Aside from breaking userspace, removing a sanity check is not a "fix".

Can't we simply have the kernel do __sev_platform_init_locked() on-demand for
SEV_PLATFORM_STATUS?  The goal with lazy initialization is defer initialization
until it's necessary so that userspace can do firmware updates.  And it's quite
clearly necessary in this case, so...