On 8/28/2023 3:56 PM, Ackerley Tng wrote:
> 1. Since the physical memory's representation is the inode and should be
> coupled to the virtual machine (as a concept, not struct kvm), should
> the binding/coupling be with the file, or the inode?
>
I've been working on Gunyah's implementation in parallel (not yet posted
anywhere). Thus far, I've coupled the virtual machine struct to the
struct file so that I can increment the file refcount when mapping the
gmem to the virtual machine.
> 2. Should struct kvm still be bound to the file/inode at gmem file
> creation time, since
>
> + struct kvm isn't a good representation of a "virtual machine"
> + we currently don't have anything that really represents a "virtual
> machine" without hardware support
>
>
> I'd also like to bring up another userspace use case that Google has:
> re-use of gmem files for rebooting guests when the KVM instance is
> destroyed and rebuilt.
>
> When rebooting a VM there are some steps relating to gmem that are
> performance-sensitive:
>
> a. Zeroing pages from the old VM when we close a gmem file/inode
> b. Deallocating pages from the old VM when we close a gmem file/inode
> c. Allocating pages for the new VM from the new gmem file/inode
> d. Zeroing pages on page allocation
>
> We want to reuse the gmem file to save re-allocating pages (b. and c.),
> and one of the two page zeroing allocations (a. or d.).
>
> Binding the gmem file to a struct kvm on creation time means the gmem
> file can't be reused with another VM on reboot. Also, host userspace is
> forced to close the gmem file to allow the old VM to be freed.
>
> For other places where files pin KVM, like the stats fd pinning vCPUs, I
> guess that matters less since there isn't much of a penalty to close and
> re-open the stats fd.
I had a 3rd question that's related to how to wire the gmem up to a
virtual machine:
I learned of a usecase to implement copy-on-write for gmem. The premise
would be to have a "golden copy" of the memory that multiple virtual
machines can map in as RO. If a virtual machine tries to write to those
pages, they get copied to a virtual machine-specific page that isn't
shared with other VMs. How do we track those pages?
Thanks,
Elliot
