Re: [RFC PATCH 9/9] kvm_main.c: handle atomic memslot update

Emanuele Giuseppe Esposito
2022-09-27T08:36:58Z

Am 27/09/2022 um 09:46 schrieb David Hildenbrand:
> On 09.09.22 12:45, Emanuele Giuseppe Esposito wrote:
>> When kvm_vm_ioctl_set_memory_region_list() is invoked, we need
>> to make sure that all memslots are updated in the inactive list
>> and then swap (preferreably only once) the lists, so that all
>> changes are visible immediately.
>>
>> The only issue is that DELETE and MOVE need to perform 2 swaps:
>> firstly replace old memslot with invalid, and then remove invalid.
>>
>
> I'm curious, how would a resize (grow/shrink) or a split be handled?
>

There are only 4 operations possible in KVM: KVM_MR_{DELETE, MOVE,
CREATE, FLAGS_ONLY}.

A resize should be implemented in QEMU as DELETE+CREATE.

Therefore a resize on memslot X will be implemented as:
First pass on the userspace operations:
invalidate memslot X;
swap_memslot_list(); // NOW it is visible to the guest

What guest sees: memslot X is invalid, so MMU keeps retrying the page fault

Second pass:
create new memslot X
delete old memslot X

What guest sees: memslot X is invalid, so MMU keeps retrying the page fault

Third pass:
swap() // 1 for each address space affected

What guest sees: new memslot replacing the old one

A split is DELETE+CREATE+CREATE, so it's the same:

First pass on the userspace operations:
invalidate memslot X;
swap_memslot_list(); // NOW it is visible to the guest

What guest sees: memslot X is invalid, so MMU keeps retrying the page fault

Second pass:
delete old memslot X
create new memslot X1
create new memslot X2

What guest sees: memslot X is invalid, so MMU keeps retrying the page fault

Third pass:
swap() // 1 for each address space affected

What guest sees: two new memslots replacing the old one