Messages in this thread | | | Date | Fri, 28 Jul 2023 17:03:33 -0700 | Subject | Re: [RFC PATCH v11 06/29] KVM: Introduce KVM_SET_USER_MEMORY_REGION2 | From | Sean Christopherson <> |
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On Fri, Jul 28, 2023, Quentin Perret wrote: > On Tuesday 18 Jul 2023 at 16:44:49 (-0700), Sean Christopherson wrote: > > --- a/include/uapi/linux/kvm.h > > +++ b/include/uapi/linux/kvm.h > > @@ -95,6 +95,16 @@ struct kvm_userspace_memory_region { > > __u64 userspace_addr; /* start of the userspace allocated memory */ > > }; > > > > +/* for KVM_SET_USER_MEMORY_REGION2 */ > > +struct kvm_userspace_memory_region2 { > > + __u32 slot; > > + __u32 flags; > > + __u64 guest_phys_addr; > > + __u64 memory_size; > > + __u64 userspace_addr; > > + __u64 pad[16]; > > Should we replace that pad[16] with: > > __u64 size; > > where 'size' is the size of the structure as seen by userspace? This is > used in other UAPIs (see struct sched_attr for example) and is a bit > more robust for future extensions (e.g. an 'old' kernel can correctly > reject a newer version of the struct with additional fields it doesn't > know about if that makes sense, etc).
"flags" serves that purpose, i.e. allows userspace to opt-in to having KVM actually consume what is currently just padding.
The padding is there mainly to simplify kernel/KVM code, e.g. the number of bytes that KVM needs to copy in is static.
But now that I think more on this, I don't know why we didn't just unconditionally bump the size of kvm_userspace_memory_region. We tried to play games with unions and overlays, but that was a mess[*].
KVM would need to do multiple uaccess reads, but that's not a big deal. Am I missing something, or did past us just get too clever and miss the obvious solution?
[*] https://lkml.kernel.org/r/Y7xrtf9FCuYRYm1q%40google.com
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