linux-kernel - Re: [PATCH v9 3/8] KVM: Add KVM_EXIT_MEMORY

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Message-ID: <Y3Uwi2lc4NDrdzML@google.com>
Date:   Wed, 16 Nov 2022 18:48:43 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Chao Peng <chao.p.peng@...ux.intel.com>,
        kvm list <kvm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
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        Paolo Bonzini <pbonzini@...hat.com>,
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        the arch/x86 maintainers <x86@...nel.org>,
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        Wei W Wang <wei.w.wang@...el.com>
Subject: Re: [PATCH v9 3/8] KVM: Add KVM_EXIT_MEMORY_FAULT exit

On Wed, Nov 16, 2022, Andy Lutomirski wrote:
> 
> 
> On Tue, Oct 25, 2022, at 8:13 AM, Chao Peng wrote:
> > diff --git a/Documentation/virt/kvm/api.rst 
> > b/Documentation/virt/kvm/api.rst
> > index f3fa75649a78..975688912b8c 100644
> > --- a/Documentation/virt/kvm/api.rst
> > +++ b/Documentation/virt/kvm/api.rst
> > @@ -6537,6 +6537,29 @@ array field represents return values. The 
> > userspace should update the return
> >  values of SBI call before resuming the VCPU. For more details on 
> > RISC-V SBI
> >  spec refer, https://github.com/riscv/riscv-sbi-doc.
> > 
> > +::
> > +
> > +		/* KVM_EXIT_MEMORY_FAULT */
> > +		struct {
> > +  #define KVM_MEMORY_EXIT_FLAG_PRIVATE	(1 << 0)
> > +			__u32 flags;
> > +			__u32 padding;
> > +			__u64 gpa;
> > +			__u64 size;
> > +		} memory;
> > +
> 
> Would it make sense to also have a field for the access type (read, write,
> execute, etc)?  I realize that shared <-> private conversion doesn't strictly
> need this, but it seems like it could be useful for logging failures and also
> for avoiding a second immediate fault if the type gets converted but doesn't
> have the right protection yet.

I don't think a separate field is necessary, that info can be conveyed via flags.
Though maybe we should go straight to a u64 for flags.  Hmm, and maybe avoid bits
0-3 so that if/when RWX info is conveyed the flags can align with
PROT_{READ,WRITE,EXEC} and the EPT flags, e.g.

	KVM_MEMORY_EXIT_FLAG_READ	(1 << 0)
	KVM_MEMORY_EXIT_FLAG_WRITE	(1 << 1)
	KVM_MEMORY_EXIT_FLAG_EXECUTE	(1 << 2)

> (Obviously, if this were changed, KVM would need the ability to report that
> it doesn't actually know the mode.)
> 
> --Andy