linux-kernel - Re: [PATCH v7 09/14] KVM: Extend the memslot to support fd-based private memory

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Date:   Wed, 3 Aug 2022 14:42:17 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Chao Peng <chao.p.peng@...ux.intel.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-api@...r.kernel.org, linux-doc@...r.kernel.org,
        qemu-devel@...gnu.org, linux-kselftest@...r.kernel.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>,
        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>,
        Shuah Khan <shuah@...nel.org>, Mike Rapoport <rppt@...nel.org>,
        Steven Price <steven.price@....com>,
        "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        luto@...nel.org, jun.nakajima@...el.com, dave.hansen@...el.com,
        ak@...ux.intel.com, david@...hat.com, aarcange@...hat.com,
        ddutile@...hat.com, dhildenb@...hat.com,
        Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>, mhocko@...e.com,
        Muchun Song <songmuchun@...edance.com>
Subject: Re: [PATCH v7 09/14] KVM: Extend the memslot to support fd-based
 private memory

On Wed, Aug 03, 2022, Chao Peng wrote:
> On Fri, Jul 29, 2022 at 07:51:29PM +0000, Sean Christopherson wrote:
> > On Wed, Jul 06, 2022, Chao Peng wrote:
> > > @@ -1332,9 +1332,18 @@ yet and must be cleared on entry.
> > >  	__u64 userspace_addr; /* start of the userspace allocated memory */
> > >    };
> > >  
> > > +  struct kvm_userspace_memory_region_ext {
> > > +	struct kvm_userspace_memory_region region;
> > > +	__u64 private_offset;
> > > +	__u32 private_fd;
> > > +	__u32 pad1;
> > > +	__u64 pad2[14];
> > > +};
> > > +
> > >    /* for kvm_memory_region::flags */
> > >    #define KVM_MEM_LOG_DIRTY_PAGES	(1UL << 0)
> > >    #define KVM_MEM_READONLY	(1UL << 1)
> > > +  #define KVM_MEM_PRIVATE		(1UL << 2)
> > 
> > Very belatedly following up on prior feedback...
> > 
> >   | I think a flag is still needed, the problem is private_fd can be safely
> >   | accessed only when this flag is set, e.g. without this flag, we can't
> >   | copy_from_user these new fields since they don't exist for previous
> >   | kvm_userspace_memory_region callers.
> > 
> > I forgot about that aspect of things.  We don't technically need a dedicated
> > PRIVATE flag to handle that, but it does seem to be the least awful soltuion.
> > We could either add a generic KVM_MEM_EXTENDED_REGION or an entirely new
> > ioctl(), e.g. KVM_SET_USER_MEMORY_REGION2, but in both approaches there's a decent
> > chance that we'll end up needed individual "this field is valid" flags anways.
> > 
> > E.g. if KVM requires pad1 and pad2 to be zero to carve out future extensions,
> > then we're right back here if some future extension needs to treat '0' as a legal
> > input.
> 
> I had such practice (always rejecting none-zero 'pad' value when
> introducing new user APIs) in other project previously, but I rarely
> see that in KVM.

Ya, KVM often uses flags to indicate the validity of a field specifically so that
KVM doesn't misinterpret a '0' from an older userspace as an intended value.