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Date:   Fri, 13 Jan 2023 22:37:39 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Chao Peng <chao.p.peng@linux.intel.com>
Cc:     Jarkko Sakkinen <jarkko@kernel.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
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        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
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        Naoya Horiguchi <naoya.horiguchi@nec.com>,
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        Andrew Morton <akpm@linux-foundation.org>,
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Subject: Re: [PATCH v10 3/9] KVM: Extend the memslot to support fd-based
 private memory
Message-ID: <Y8HdMzlNFhFwlkGS@google.com>
References: <20221202061347.1070246-1-chao.p.peng@linux.intel.com>
 <20221202061347.1070246-4-chao.p.peng@linux.intel.com>
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 <20230106094000.GA2297836@chaop.bj.intel.com>
 <Y7xrtf9FCuYRYm1q@google.com>
 <20230110091432.GA2441264@chaop.bj.intel.com>
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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);							\
	})

> Then we will use kvm_userspace_memory_region2 as the KVM internal alias,
> right?

Yep.

> I see similar examples use different functions to handle different versions
> but it does look easier if we use alias for this function.
> 
> > 
> > It feels absolutely ridiculous, but I think the best option is to do:
> > 
> > #define KVM_SET_USER_MEMORY_REGION2 _IOW(KVMIO, 0x49, \
> > 					 struct kvm_userspace_memory_region2)
> 
> Just interesting, is 0x49 a safe number we can use? 

Yes?  So long as its not used by KVM, it's safe.  AFAICT, it's unused.