Re: [PATCH v2 1/9] KVM: x86: Add AMD SEV specific Hypercall3

[Ashish Kalra]

> On Thu, Jan 07, 2021 at 01:34:14AM +0000, Ashish Kalra wrote:
> > Hello Steve,
> > 
> > My thoughts here ...
> > 
> > On Wed, Jan 06, 2021 at 05:01:33PM -0800, Steve Rutherford wrote:
> > > Avoiding an rbtree for such a small (but unstable) list seems correct.
> > > 
> > 
> > I agree.
> > 
> > > For the unencrypted region list strategy, the only questions that I
> > > have are fairly secondary.
> > > - How should the kernel upper bound the size of the list in the face
> > > of malicious guests, but still support large guests? (Something
> > > similar to the size provided in the bitmap API would work).
> > > - What serialization format should be used for the ioctl API?
> > > (Usermode could send down a pointer to a user region and a size. The
> > > kernel could then populate that with an array of structs containing
> > > bases and limits for unencrypted regions.)
> > > - How will the kernel tag a guest as having exceeded its maximum list
> > > size, in order to indicate that the list is now incomplete? (Track a
> > > poison bit, and send it up when getting the serialized list of
> > > regions).
> > > 
> > > In my view, there are two main competitors to this strategy:
> > > - (Existing) Bitmap API
> > > - A guest memory donation based model
> > > 
> > > The existing bitmap API avoids any issues with growing too large,
> > > since it's size is predictable.
> > > 
> > > To elaborate on the memory donation based model, the guest could put
> > > an encryption status data structure into unencrypted guest memory, and
> > > then use a hypercall to inform the host where the base of that
> > > structure is located. The main advantage of this is that it side steps
> > > any issues around malicious guests causing large allocations.
> > > 
> > > The unencrypted region list seems very practical. It's biggest
> > > advantage over the bitmap is how cheap it will be to pass the
> > > structure up from the kernel. A memory donation based model could
> > > achieve similar performance, but with some additional complexity.
> > > 
> > > Does anyone view the memory donation model as worth the complexity?
> > > Does anyone think the simplicity of the bitmap is a better tradeoff
> > > compared to an unencrypted region list?
> > 
> > One advantage in sticking with the bitmap is that it maps very nicely to
> > the dirty bitmap page tracking logic in KVM/Qemu. The way Brijesh
> > designed and implemented it is very similar to dirty page bitmap tracking
> > and syncing between KVM and Qemu. The same logic is re-used for the page
> > encryption bitmap which means quite mininal changes and code resuse in
> > Qemu. 
> > 
> > Any changes to the backing data structure, will require additional
> > mapping logic to be added to Qemu.
> > 
> > This is one advantage in keeping the bitmap logic.
> > 

So if nobody is in favor of keeping the (current) bitmap logic, we will
move to the unencrypted region list approach.

Thanks,
Ashish