On 25.04.25 05:44, H. Peter Anvin wrote:

On 4/24/25 18:15, H. Peter Anvin wrote:

quoted

On 4/24/25 01:14, Jürgen Groß wrote:

quoted

quoted

Actually, that is how we get this patch with the existing alternatives
infrastructure.  And we took a step further to also remove the pv_ops
MSR APIs...

And this is what I'm questioning. IMHO this approach is adding more
code by removing the pv_ops MSR_APIs just because "pv_ops is bad". And
I believe most refusal of pv_ops is based on no longer valid reasoning.

pvops are a headache because it is effectively a secondary alternatives 
infrastructure that is incompatible with the alternatives one...

quoted

quoted

It looks to me that you want to add a new facility to the alternatives
infrastructure first?

Why would we need a new facility in the alternatives infrastructure?

I'm not sure what Xin means with "facility", but a key motivation for this is to:

a. Avoid using the pvops for MSRs when on the only remaining user thereof 
(Xen) is only using it for a very small subset of MSRs and for the rest it is 
just overhead, even for Xen;

b. Being able to do wrmsrns immediate/wrmsrns/wrmsr and rdmsr immediate/ rdmsr 
alternatives.

Of these, (b) is by far the biggest motivation. The architectural direction 
for supervisor states is to avoid ad hoc and XSAVES ISA and instead use MSRs. 
The immediate forms are expected to be significantly faster, because they make 
the MSR index available at the very beginning of the pipeline instead of at a 
relatively late stage.

Note that to support the immediate forms, we *must* do these inline, or the 
const-ness of the MSR index -- which applies to by far the vast majority of MSR 
references -- gets lost. pvops does exactly that.

Furthermore, the MSR immediate instructions take a 64-bit number in a single 
register; as these instructions are by necessity relatively long, it makes sense 
for the alternative sequence to accept a 64-bit input register and do the %eax/ 
%edx shuffle in the legacy fallback code... we did a bunch of experiments to see 
what made most sense.

Yes, I understand that.

And I'm totally in favor of Xin's rework of the MSR low level functions.

Inlining the MSR access instructions with pv_ops should not be very
complicated. We do that with other instructions (STI/CLI, PTE accesses)
today, so this is no new kind of functionality.

I could have a try writing a patch achieving that, but I would only start
that work in case you might consider taking it instead of Xin's patch
removing the pv_ops usage for rdmsr/wrmsr. In case it turns out that my
version results in more code changes than Xin's patch, I'd be fine to drop
my patch, of course.


Juergen