Re: [PATCH for-8.2 0/2] target/riscv: add zicntr and zihpm flags

[Conor Dooley <conor@kernel.org>]

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On Mon, Jul 17, 2023 at 08:11:09PM -0300, Daniel Henrique Barboza wrote:
> On 7/17/23 19:33, Conor Dooley wrote:

> > On Mon, Jul 17, 2023 at 06:54:17PM -0300, Daniel Henrique Barboza wrote:
> > > Hi,
> > > 
> > > I decided to include flags for both timer/counter extensions to make it
> > > easier for us later on when dealing with the RVA22 profile (which
> > > includes both).
> > > 
> > > The features were already implemented by Atish Patra some time ago, but
> > > back then these 2 extensions weren't introduced yet. This means that,
> > > aside from extra stuff in riscv,isa FDT no other functional changes were
> > > made.
> > > 
> > > Both are defaulted to 'true' since QEMU already implements both
> > > features, but the flag can be disabled if Zicsr isn't present or, in
> > > the case of zihpm, if pmu_num = 0.
> > 
> > Out of curiosity, since you are allowing them to be disabled, how do you
> > intend to communicate to a guest that zicsr or zihpm are not present?
> 
> At this point I'd say that existing guests are using other ways of checking
> if these timers and counters are available.

Or they just assume they're there as part of their baseline requirements
;)

> After this patches OSes can confirm
> if these timers are available via riscv,isa, but they can't assume that
> they are not available if riscv,isa doesn't display them.
> 
> There's a chance that guests will continue ignoring these 2 extensions regardless
> of whether the platform exposes them or not.
> 
> > 
> > > This means that,
> > > aside from extra stuff in riscv,isa FDT no other functional changes were
> > > made.
> > 
> > This is barely a "functional" change either, as the presence of these
> > extensions has to be assumed, whether they appear in riscv,isa or not :/
> 
> It's more of an organizational change for the sake of QEMU internals because the
> RVA22 profile happens to include zicntr and zihpm as mandatory extensions. It's
> easier to add the flags than to document why we're claiming RVA22 support but
> aren't displaying these 2 in riscv,isa.

Possibly you should call out ACPI here too, since that does not suffer
from the same issues as riscv,isa in DT, and putting zicntr/zihpm et al
in the ISA string there is needed.

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