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

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Message-ID: <a121e766-900d-2135-1516-e1d3ba716834@maciej.szmigiero.name>
Date:   Tue, 25 Jan 2022 21:20:39 +0100
From:   "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
To:     Chao Peng <chao.p.peng@...ux.intel.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>
Cc:     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 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.

Thanks,
Maciej