linux-kernel - Re: [PATCH v5 05/13] KVM: Extend the memslot to support fd-based private memory

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Message-ID: <YkIvEeC3/lgKTLPt@google.com>
Date:   Mon, 28 Mar 2022 21:56:33 +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, 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>,
        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>,
        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
Subject: Re: [PATCH v5 05/13] KVM: Extend the memslot to support fd-based
 private memory

On Thu, Mar 10, 2022, Chao Peng wrote:
> Extend the memslot definition to provide fd-based private memory support
> by adding two new fields (private_fd/private_offset). The memslot then
> can maintain memory for both shared pages and private pages in a single
> memslot. Shared pages are provided by existing userspace_addr(hva) field
> and private pages are provided through the new private_fd/private_offset
> fields.
> 
> Since there is no 'hva' concept anymore for private memory so we cannot
> rely on get_user_pages() to get a pfn, instead we use the newly added
> memfile_notifier to complete the same job.
> 
> This new extension is indicated by a new flag KVM_MEM_PRIVATE.
> 
> Signed-off-by: Yu Zhang <yu.c.zhang@...ux.intel.com>
> Signed-off-by: Chao Peng <chao.p.peng@...ux.intel.com>
> ---
>  Documentation/virt/kvm/api.rst | 37 +++++++++++++++++++++++++++-------
>  include/linux/kvm_host.h       |  7 +++++++
>  include/uapi/linux/kvm.h       |  8 ++++++++
>  3 files changed, 45 insertions(+), 7 deletions(-)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 3acbf4d263a5..f76ac598606c 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -1307,7 +1307,7 @@ yet and must be cleared on entry.
>  :Capability: KVM_CAP_USER_MEMORY
>  :Architectures: all
>  :Type: vm ioctl
> -:Parameters: struct kvm_userspace_memory_region (in)
> +:Parameters: struct kvm_userspace_memory_region(_ext) (in)
>  :Returns: 0 on success, -1 on error
>  
>  ::
> @@ -1320,9 +1320,17 @@ 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;

Peeking ahead, the partial switch to the _ext variant is rather gross.  I would
prefer that KVM use an entirely different, but binary compatible, struct internally.
And once the kernel supports C11[*], I'm pretty sure we can make the "region" in
_ext an anonymous struct, and make KVM's internal struct a #define of _ext.  That
should minimize the churn (no need to get the embedded "region" field), reduce
line lengths, and avoid confusion due to some flows taking the _ext but others
dealing with only the "base" struct.

Maybe kvm_user_memory_region or kvm_user_mem_region?  Though it's tempting to be
evil and usurp the old kvm_memory_region :-)

E.g. pre-C11 do

struct kvm_userspace_memory_region_ext {
	struct kvm_userspace_memory_region region;
	__u64 private_offset;
	__u32 private_fd;
	__u32 padding[5];
};

#ifdef __KERNEL__
struct kvm_user_mem_region {
	__u32 slot;
	__u32 flags;
	__u64 guest_phys_addr;
	__u64 memory_size; /* bytes */
	__u64 userspace_addr; /* start of the userspace allocated memory */
	__u64 private_offset;
	__u32 private_fd;
	__u32 padding[5];
};
#endif

and then post-C11 do

struct kvm_userspace_memory_region_ext {
#ifdef __KERNEL__
	struct kvm_userspace_memory_region region;
#else
	struct kvm_userspace_memory_region;
#endif
	__u64 private_offset;
	__u32 private_fd;
	__u32 padding[5];
};

#ifdef __KERNEL__
#define kvm_user_mem_region kvm_userspace_memory_region_ext
#endif

[*] https://lore.kernel.org/all/20220301145233.3689119-1-arnd@kernel.org

> +	__u64 private_offset;
> +	__u32 private_fd;
> +	__u32 padding[5];
> +};