On 28/01/21 20:26, David Woodhouse wrote:
Honestly, I don't even care about reading it out except for long_mode
which the kernel *does* infer for itself when the MSR is used to fill
in the hypercall page.
What about VM migration?
The VMM needs to know all these things anyway. It's not like it can
*forget* where each vCPU's vcpu_info is, then just ask the kernel when
it's time to orchestrate a migration.
Yeah it may not be particularly useful but it's not hard to write the
code and it's easier to document.
I suppose *maybe* the upcall vector is an exception; the VMM *could*
forget that if it really wanted to, then ask the kernel for it on
migration. But we don't because that seems pointless.
But then it *theoretically* could have a sparse bitmap of "this feature
but not the next feature", even though that would probably never happen
in practice without weird selective backports. But it's still an icky
API. And frankly I could live without the 'get' for any of these except
LONG_MODE which the kernel might actually flip for us. The rest could
be write-only for all I care.
I don't think it's that icky, the alternative would be a KVM_GET_ONE_REG
like architecture that just returns a u64 but the bitmap works too.
It's not hard to write the code.
Paolo
