linux-kernel - Re: [PATCH v10 3/9] KVM: Extend the memslot to support fd-based private memory

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Date:   Tue, 17 Jan 2023 20:42:14 +0800
From:   Chao Peng <chao.p.peng@...ux.intel.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Jarkko Sakkinen <jarkko@...nel.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-arch@...r.kernel.org,
        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>,
        Arnd Bergmann <arnd@...db.de>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Miaohe Lin <linmiaohe@...wei.com>, 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>, tabba@...gle.com,
        Michael Roth <michael.roth@....com>, mhocko@...e.com,
        wei.w.wang@...el.com
Subject: Re: [PATCH v10 3/9] KVM: Extend the memslot to support fd-based
 private memory

On Fri, Jan 13, 2023 at 10:37:39PM +0000, Sean Christopherson wrote:
> On Tue, Jan 10, 2023, Chao Peng wrote:
> > On Mon, Jan 09, 2023 at 07:32:05PM +0000, Sean Christopherson wrote:
> > > On Fri, Jan 06, 2023, Chao Peng wrote:
> > > > On Thu, Jan 05, 2023 at 11:23:01AM +0000, Jarkko Sakkinen wrote:
> > > > > On Fri, Dec 02, 2022 at 02:13:41PM +0800, Chao Peng wrote:
> > > > > > To make future maintenance easy, internally use a binary compatible
> > > > > > alias struct kvm_user_mem_region to handle both the normal and the
> > > > > > '_ext' variants.
> > > > > 
> > > > > Feels bit hacky IMHO, and more like a completely new feature than
> > > > > an extension.
> > > > > 
> > > > > Why not just add a new ioctl? The commit message does not address
> > > > > the most essential design here.
> > > > 
> > > > Yes, people can always choose to add a new ioctl for this kind of change
> > > > and the balance point here is we want to also avoid 'too many ioctls' if
> > > > the functionalities are similar.  The '_ext' variant reuses all the
> > > > existing fields in the 'normal' variant and most importantly KVM
> > > > internally can reuse most of the code. I certainly can add some words in
> > > > the commit message to explain this design choice.
> > > 
> > > After seeing the userspace side of this, I agree with Jarkko; overloading
> > > KVM_SET_USER_MEMORY_REGION is a hack.  E.g. the size validation ends up being
> > > bogus, and userspace ends up abusing unions or implementing kvm_user_mem_region
> > > itself.
> > 
> > How is the size validation being bogus? I don't quite follow.
> 
> The ioctl() magic embeds the size of the payload (struct kvm_userspace_memory_region
> in this case) in the ioctl() number, and that information is visible to userspace
> via _IOCTL_SIZE().  Attempting to take a larger size can mess up sanity checks,
> e.g. KVM selftests get tripped up on this assert if KVM_SET_USER_MEMORY_REGION is
> passed an "extended" struct.
> 
> 	#define kvm_do_ioctl(fd, cmd, arg)						\
> 	({										\
> 		kvm_static_assert(!_IOC_SIZE(cmd) || sizeof(*arg) == _IOC_SIZE(cmd));	\
> 		ioctl(fd, cmd, arg);							\
> 	})

Got it. Thanks for the explanation.

Chao