linux-kernel - Re: [PATCH v4 12/12] KVM: Expose KVM_MEM

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Message-ID: <20220224080739.GA6672@chaop.bj.intel.com>
Date:   Thu, 24 Feb 2022 16:07:39 +0800
From:   Chao Peng <chao.p.peng@...ux.intel.com>
To:     "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
Cc:     Yu Zhang <yu.c.zhang@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jonathan Corbet <corbet@....net>,
        Sean Christopherson <seanjc@...gle.com>,
        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>, kvm@...r.kernel.org,
        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>,
        "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, qemu-devel@...gnu.org
Subject: Re: [PATCH v4 12/12] KVM: Expose KVM_MEM_PRIVATE

On Wed, Feb 23, 2022 at 07:32:37PM +0100, Maciej S. Szmigiero wrote:
> On 23.02.2022 13:00, Chao Peng wrote:
> > On Tue, Feb 22, 2022 at 02:16:46AM +0100, Maciej S. Szmigiero wrote:
> > > On 17.02.2022 14:45, Chao Peng wrote:
> > > > On Tue, Jan 25, 2022 at 09:20:39PM +0100, Maciej S. Szmigiero wrote:
> > > > > On 18.01.2022 14:21, Chao Peng wrote:
> > > > > > KVM_MEM_PRIVATE is not exposed by default but architecture code can turn
> > > > > > on it by implementing kvm_arch_private_memory_supported().
> > > > > > 
> > > > > > Also private memslot cannot be movable and the same file+offset can not
> > > > > > be mapped into different GFNs.
> > > > > > 
> > > > > > Signed-off-by: Yu Zhang <yu.c.zhang@...ux.intel.com>
> > > > > > Signed-off-by: Chao Peng <chao.p.peng@...ux.intel.com>
> > > > > > ---
> > > > > (..)
> > > > > >     static bool kvm_check_memslot_overlap(struct kvm_memslots *slots, int id,
> > > > > > -				      gfn_t start, gfn_t end)
> > > > > > +				      struct file *file,
> > > > > > +				      gfn_t start, gfn_t end,
> > > > > > +				      loff_t start_off, loff_t end_off)
> > > > > >     {
> > > > > >     	struct kvm_memslot_iter iter;
> > > > > > +	struct kvm_memory_slot *slot;
> > > > > > +	struct inode *inode;
> > > > > > +	int bkt;
> > > > > >     	kvm_for_each_memslot_in_gfn_range(&iter, slots, start, end) {
> > > > > >     		if (iter.slot->id != id)
> > > > > >     			return true;
> > > > > >     	}
> > > > > > +	/* Disallow mapping the same file+offset into multiple gfns. */
> > > > > > +	if (file) {
> > > > > > +		inode = file_inode(file);
> > > > > > +		kvm_for_each_memslot(slot, bkt, slots) {
> > > > > > +			if (slot->private_file &&
> > > > > > +			     file_inode(slot->private_file) == inode &&
> > > > > > +			     !(end_off <= slot->private_offset ||
> > > > > > +			       start_off >= slot->private_offset
> > > > > > +					     + (slot->npages >> PAGE_SHIFT)))
> > > > > > +				return true;
> > > > > > +		}
> > > > > > +	}
> > > > > 
> > > > > That's a linear scan of all memslots on each CREATE (and MOVE) operation
> > > > > with a fd - we just spent more than a year rewriting similar linear scans
> > > > > into more efficient operations in KVM.
> > > > 
> (..)
> > > > So linear scan is used before I can find a better way.
> > > 
> > > Another option would be to simply not check for overlap at add or move
> > > time, declare such configuration undefined behavior under KVM API and
> > > make sure in MMU notifiers that nothing bad happens to the host kernel
> > > if it turns out somebody actually set up a VM this way (it could be
> > > inefficient in this case, since it's not supposed to ever happen
> > > unless there is a bug somewhere in the userspace part).
> > 
> > Specific to TDX case, SEAMMODULE will fail the overlapping case and then
> > KVM prints a message to the kernel log. It will not cause any other side
> > effect, it does look weird however. Yes warn that in the API document
> > can help to some extent.
> 
> So for the functionality you are adding this code for (TDX) this scan
> isn't necessary and the overlapping case (not supported anyway) is safely
> handled by the hardware (or firmware)?

Yes, it will be handled by the firmware.

> Then I would simply remove the scan and, maybe, add a comment instead
> that the overlap check is done by the hardware.

Sure.

> 
> By the way, if a kernel log message could be triggered by (misbehaving)
> userspace then it should be rate limited (if it isn't already).

Thanks for mention.

Chao
> 
> > Thanks,
> > Chao
> 
> Thanks,
> Maciej