Re: [PATCH v7 09/14] KVM: Extend the memslot to support fd-based private memory

Chao Peng
2022-08-03T10:13:37Z

On Fri, Jul 29, 2022 at 07:51:29PM +0000, Sean Christopherson wrote:
> On Wed, Jul 06, 2022, Chao Peng wrote:
> > @@ -1332,9 +1332,18 @@ 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;
> > + __u64 private_offset;
> > + __u32 private_fd;
> > + __u32 pad1;
> > + __u64 pad2[14];
> > +};
> > +
> > /* for kvm_memory_region::flags */
> > #define KVM_MEM_LOG_DIRTY_PAGES (1UL << 0)
> > #define KVM_MEM_READONLY (1UL << 1)
> > + #define KVM_MEM_PRIVATE (1UL << 2)
>
> Very belatedly following up on prior feedback...
>
> | I think a flag is still needed, the problem is private_fd can be safely
> | accessed only when this flag is set, e.g. without this flag, we can't
> | copy_from_user these new fields since they don't exist for previous
> | kvm_userspace_memory_region callers.
>
> I forgot about that aspect of things. We don't technically need a dedicated
> PRIVATE flag to handle that, but it does seem to be the least awful soltuion.
> We could either add a generic KVM_MEM_EXTENDED_REGION or an entirely new
> ioctl(), e.g. KVM_SET_USER_MEMORY_REGION2, but in both approaches there's a decent
> chance that we'll end up needed individual "this field is valid" flags anways.
>
> E.g. if KVM requires pad1 and pad2 to be zero to carve out future extensions,
> then we're right back here if some future extension needs to treat '0' as a legal
> input.

I had such practice (always rejecting none-zero 'pad' value when
introducing new user APIs) in other project previously, but I rarely
see that in KVM.

>
> TL;DR: adding KVM_MEM_PRIVATE still seems like the best approach.
>
> > @@ -4631,14 +4658,35 @@ static long kvm_vm_ioctl(struct file *filp,
> > break;
> > }
> > case KVM_SET_USER_MEMORY_REGION: {
> > - struct kvm_userspace_memory_region kvm_userspace_mem;
> > + struct kvm_user_mem_region mem;
> > + unsigned long size;
> > + u32 flags;
> > +
> > + kvm_sanity_check_user_mem_region_alias();
> > +
> > + memset(&mem, 0, sizeof(mem));
> >
> > r = -EFAULT;
> > - if (copy_from_user(&kvm_userspace_mem, argp,
> > - sizeof(kvm_userspace_mem)))
> > +
> > + if (get_user(flags,
> > + (u32 __user *)(argp + offsetof(typeof(mem), flags))))
> > + goto out;
>
>
> Indentation is funky. It's hard to massage this into something short and
> readable What about capturing the offset separately? E.g.
>
> struct kvm_user_mem_region mem;
> unsigned int flags_offset = offsetof(typeof(mem), flags));
> unsigned long size;
> u32 flags;
>
> kvm_sanity_check_user_mem_region_alias();
>
> memset(&mem, 0, sizeof(mem));
>
> r = -EFAULT;
> if (get_user(flags, (u32 __user *)(argp + flags_offset)))
> goto out;
>
> But this can actually be punted until KVM_MEM_PRIVATE is fully supported. As of
> this patch, KVM doesn't read the extended size, so I believe the diff for this
> patch can simply be:

Looks good to me, Thanks.

Chao
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index da263c370d00..5194beb7b52f 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -4640,6 +4640,10 @@ static long kvm_vm_ioctl(struct file *filp,
> sizeof(kvm_userspace_mem)))
> goto out;
>
> + r = -EINVAL;
> + if (mem.flags & KVM_MEM_PRIVATE)
> + goto out;
> +
> r = kvm_vm_ioctl_set_memory_region(kvm, &kvm_userspace_mem);
> break;
> }