Re: [PATCH v1 1/2] KVM: PPC: Book3S HV: Add support for H_RPT_INVALIDATE (nested case only)

[Paul Mackerras <paulus@ozlabs.org>]

On Mon, Oct 19, 2020 at 04:56:41PM +0530, Bharata B Rao wrote:
> Implements H_RPT_INVALIDATE hcall and supports only nested case
> currently.
> 
> A KVM capability KVM_CAP_RPT_INVALIDATE is added to indicate the
> support for this hcall.

I have a couple of questions about this patch:

1. Is this something that is useful today, or is it something that may
become useful in the future depending on future product plans?  In
other words, what advantage is there to forcing L2 guests to use this
hcall instead of doing tlbie themselves?

2. Why does it need to be added to the default-enabled hcall list?

There is a concern that if this is enabled by default we could get the
situation where a guest using it gets migrated to a host that doesn't
support it, which would be bad.  That is the reason that all new
things like this are disabled by default and only enabled by userspace
(i.e. QEMU) in situations where we can enforce that it is available on
all hosts to which the VM might be migrated.

Thanks,
Paul.