Re: [PATCH v3 01/17] KVM: Fix arguments to kvm_{un,}map_gfn()

[Vitaly Kuznetsov <vkuznets@redhat.com>]

David Woodhouse <dwmw2@infradead.org> writes:

> On 14 December 2020 21:41:23 GMT, Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>>>Your change is correct but I'm not sure that it's entirely clear that
>>kvm_map_gfn() implicitly uses 'as_id=0' and I don't even see a comment
>>about the fact :-(
>
> Isn't that true of all the kvm_read_guest and kvm_write_guest
> functions and indeed of kvm_memslots() itself?

Yes, sadly. Multiple address spaces support was added to KVM as a
generic feature but the only use-case for it at this moment is SMM on
x86 which is 'special', i.e. currently there's only one user for 
kvm_map_gfn()/kvm_unmap_gfn() which is steal time accounting and it's
not easy to come up with a use-case when this PV feature needs to be
used from SMM. On the other hand, if we try using multiple address
spaces in KVM for e.g. emulating something like Hyper-V VSM, it becomes
unclear which address space id needs to be used even for steal
time. To be entirely correct, we probably need to remember as_id which
was active when steal time was enabled and stick to it later when we
want to update the data. If we do that, kvm_map_gfn() will lose its only
user.

All the above doesn't make your patch incorrect of course, I just used
it to express my observation that we seem to be using as_id=0 blindly
and the API we have contributes to that.

-- 
Vitaly