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Date:   Fri, 8 Apr 2022 17:01:21 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Peter Gonda <pgonda@google.com>
Cc:     kvm list <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Anup Patel <anup@brainfault.org>, Will Deacon <will@kernel.org>
Subject: Re: [PATCH v4.1] KVM, SEV: Add KVM_EXIT_SHUTDOWN metadata for SEV-ES
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+Anup and Will

On Fri, Apr 08, 2022, Peter Gonda wrote:
> On Thu, Apr 7, 2022 at 8:55 PM Sean Christopherson <seanjc@google.com> wrote:
> >
> > On Thu, Apr 07, 2022, Peter Gonda wrote:
> > > If an SEV-ES guest requests termination, exit to userspace with
> > > KVM_EXIT_SYSTEM_EVENT and a dedicated SEV_TERM type instead of -EINVAL
> > > so that userspace can take appropriate action.
> > >
> > > See AMD's GHCB spec section '4.1.13 Termination Request' for more details.
> >
> > Maybe it'll be obvious by the lack of compilation errors, but the changelog should
> > call out the flags => ndata+data shenanigans, otherwise this looks like ABI breakage.
> 
> Hmm I am not sure we can do this change anymore given that we have two
> call sites using 'flags'
> 
> arch/arm64/kvm/psci.c:184
> arch/riscv/kvm/vcpu_sbi.c:97
> 
> I am not at all familiar with ARM and RISC-V but some quick reading
> tells me these archs also require 64-bit alignment on their 64-bit
> accesses. If thats correct, should I fix this call sites up by
> proceeding with this ndata + data[] change and move whatever they are
> assigning to flags into data[0] like I am doing here? It looks like
> both of these changes are not in a kernel release so IIUC we can still
> fix the ABI here?

Yeah, both came in for v5.18.  Given that there will be multiple paths that need
to set data, it's worth adding a common helper to the dirty work.

Anup and Will,

system_event.flags is broken (at least on x86) due to the prior 'type' field not
being propery padded, e.g. userspace will read/write garbage if the userspace
and kernel compilers pad structs differently.

		struct {
			__u32 type;
			__u64 flags;
		} system_event;

Our plan to unhose this is to change the struct as follows and use bit 31 in the
'type' to indicate that ndata+data are valid.

		struct {
                        __u32 type;
			__u32 ndata;
			__u64 data[16];
                } system_event;

Any objection to updating your architectures to use a helper to set the bit and
populate ndata+data accordingly?  It'll require a userspace update, but v5.18
hasn't officially released yet so it's not kinda sort not ABI breakage.