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

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Date:   Thu, 17 Nov 2022 21:42:50 +0800
From:   Chao Peng <chao.p.peng@...ux.intel.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Andy Lutomirski <luto@...nel.org>, kvm list <kvm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-arch@...r.kernel.org, Linux API <linux-api@...r.kernel.org>,
        linux-doc@...r.kernel.org, qemu-devel@...gnu.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Hugh Dickins <hughd@...gle.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
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        Steven Price <steven.price@....com>,
        "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
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        Yu Zhang <yu.c.zhang@...ux.intel.com>,
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        "Nakajima, Jun" <jun.nakajima@...el.com>,
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        Fuad Tabba <tabba@...gle.com>,
        Michael Roth <michael.roth@....com>,
        Michal Hocko <mhocko@...e.com>,
        Muchun Song <songmuchun@...edance.com>,
        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 at 06:48:43PM +0000, Sean Christopherson wrote:
> 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.

Yeah, I can do that.

> 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.

You mean avoiding bits 0-2, right, bit3 is not so special and can be used
for KVM_MEMORY_EXIT_FLAG_PRIVATE.

Chao
> 
> 	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