On 10/5/23 19:06, Jim Mattson wrote:
On Thu, Oct 5, 2023 at 9:39 AM Paolo Bonzini<pbonzini@xxxxxxxxxx> wrote:
I agree with Jim that it would be nice to have some bits from Intel, and
some bits from AMD, that current processors always return as 1. Future
processors can change those to 0 as desired.
That's not quite what I meant.
I'm suggesting a leaf devoted to single bit negative features. If a
bit is set in hardware, it means that something has been taken away.
Hypervisors don't need to know exactly what was taken away. For this
leaf only, hypervisors will always pass through a non-zero bit, even
if they have know idea what it means.
Understood, but I'm suggesting that these might even have the right
polarity: if a bit is set it means that something is there and might not
in the future, even if we don't know exactly what. We can pass through
the bit, we can AND bits across the migration pool to define what to
pass to the guest, we can force-set the leaves to zero (feature
removed). Either way, the point is to group future defeatures together.
That said, these bits are only for documentation/debugging purposes
anyway. So I like the idea because it would educate the architects
about this issue, more than because it is actually useful...
Paolo
