[RFC PATCH 29/32] KVM: arm64: Pass hypercalls to userspace

[James Morse]

Hi Oliver,

On 03/02/2023 21:08, Oliver Upton wrote:
> On Fri, Feb 03, 2023 at 01:50:40PM +0000, James Morse wrote:
>> From: Jean-Philippe Brucker <jean-philippe at linaro.org>
>>
>> When capability KVM_CAP_ARM_HVC_TO_USER is available, userspace can
>> request to handle all hypercalls that aren't handled by KVM.

> I would very much prefer we not go down this route. This capability
> effectively constructs an ABI out of what KVM presently does not
> implement. What would happen if KVM decides to implement a new set
> of hypercalls later down the road that were previously forwarded to
> userspace?

The user-space support would never get called. If we have a wild-west allocation of IDs in
this area we have bigger problems. I'd hope in this example it would be a VMM or an
in-kernel implementation of the same feature.

When I floated something like this before for supporting SDEI in guests, Christoffer
didn't like tie-ing KVM to SMC-CC - hence the all or nothing.

Since then we've had things like Spectre, which I don't think the VMM should
ever be allowed to handle, which makes the whole thing much murkier.


> Instead of a catch-all I think we should take the approach of having
> userspace explicitly request which hypercalls should be forwarded to
> userspace. I proposed something similar [1], but never got around to
> respinning it (oops).

> Let me dust those patches off and align with Marc's suggestions.
> 
> [1]: https://lore.kernel.org/kvmarm/20221110015327.3389351-1-oliver.upton@linux.dev/

I've no problem with doing it like this. This approach was based on Christoffer's previous
feedback, but the world has changed since then.

Let me know if you want me to re-spin that series - I need to get this into some
shape next week for Salil to look at the Qemu changes, as I can't test the whole thing
until that is done.



Thanks,

James