Re: [PATCH 0/6] riscv: RVA22U64 profile support

[Alistair Francis]

On Fri, Sep 29, 2023 at 10:54 PM Daniel P. Berrangé <berrange@redhat.com> wrote:
>
> On Fri, Sep 29, 2023 at 09:49:47AM -0300, Daniel Henrique Barboza wrote:
> >
> >
> > On 9/29/23 08:55, Daniel P. Berrangé wrote:
> > > On Fri, Sep 29, 2023 at 08:29:08AM -0300, Daniel Henrique Barboza wrote:
> > > >
> > > >
> > > > On 9/29/23 07:46, Daniel P. Berrangé wrote:
> > > > > On Tue, Sep 26, 2023 at 04:49:44PM -0300, Daniel Henrique Barboza 
> > > > > wrote:
> > > > > > Based-on: 20230926183109.165878-1-dbarboza@ventanamicro.com
> > > > > > ("[PATCH 0/2] riscv: add extension properties for all cpus")
> > > > > >
> > > > > > Hi,
> > > > > >
> > > > > > These patches implements the base profile support for qemu-riscv 
> > > > > > and the
> > > > > > first profile, RVA22U64.
> > > > > >
> > > > > > As discussed in this thread [1] we're aiming for a flag that 
> > > > > > enables all
> > > > > > mandatory extensions of a profile. Optional extensions were left 
> > > > > > behind
> > > > > > and must be enabled by hand if desired. Since this is the first 
> > > > > > profile
> > > > > > we're adding, we'll need to add the base framework as well.
> > > > > >
> > > > > > The RVA22U64 profile was chosen because qemu-riscv implements all 
> > > > > > its
> > > > > > extensions, both mandatory and optional. That includes 'zicntr' and
> > > > > > 'zihpm', which we support for awhile but aren't adverting to 
> > > > > > userspace.
> > > > > >
> > > > > > Other design decisions made:
> > > > > >
> > > > > > - disabling a profile flag does nothing, i.e. we won't mass disable
> > > > > >     mandatory extensions of the rva22U64 profile if the user sets
> > > > > >     rva22u64=false;
> > > > >
> > > > > Why shouldn't this be allowed ?
> > > > >
> > > > > IIUC, a profile is syntactic sugar for a group of features. If
> > > > > we can disable individual features explicitly, why should we
> > > > > not allow use of the profile as sugar to disable them en-mass ?
> > > >
> > > > In theory there's no harm in allowing mass disabling of extensions but, 
> > > > given
> > > > it's a whole profile, we would end up disabling most/all CPU extensions 
> > > > and
> > > > the guest would do nothing.
> > >
> > > True, that is just user error though.  They could disable a profile
> > > and then manually re-enable individual features, and thus get a
> > > working system.
> > >
> > > > There is a thread in the ML:
> > > >
> > > > https://lore.kernel.org/qemu-riscv/CABJz62NyVNu4Z1qmCG7MyJkGG_9yWxjUFHHWjmoQEP6unRrHNA@mail.gmail.com/
> > > >
> > > > Where we discussed the possibility of having a minimal CPU extension 
> > > > set. We didn't
> > > > reach a consensus because the definition of "minimal CPU extension set" 
> > > > vary between
> > > > OSes (Linux requires IMAFD, FreeBSD might require something differ).
> > > >
> > > > Assuming we reach a consensus on what a minimal set is, we could allow 
> > > > disabling mass
> > > > extensions via probile but keeping this minimal set, for example. At 
> > > > very least we
> > > > shouldn't allow users to disable 'I' because that would kill the CPU, 
> > > > so RV64I is
> > > > the minimum set that I would assume for now.
> > >
> > > I'd probably just call that user error too.
> > >
> > > > >
> > > > > TL;DR: feature groups are pretty error prone if more than
> > > > > one is listed by the user, or they're combined with individual
> > > > > features.
> > > > >
> > > > > >
> > > > > > - profile support for vendor CPUs consists into checking if the CPU
> > > > > >     happens to have the mandatory extensions required for it. In 
> > > > > > case it
> > > > > >     doesn't we'll error out. This is done to follow the same 
> > > > > > prerogative
> > > > > >     we always had of not allowing extensions being enabled for 
> > > > > > vendor
> > > > > >     CPUs;
> > > > >
> > > > > Why shouldn't this be allowed ?
> > > >
> > > > There's no technical reason to not allow it. The reason it's forbid is 
> > > > to be
> > > > closer to what the real hardware would do. E.g. the real hardware 
> > > > doesn't allow
> > > > users to enable Vector if the hardware doesn't support it. Vendor CPUs 
> > > > also has
> > > > a privileged spec restriction as well, so if a CPU is running in an 
> > > > older spec
> > > > it can't enable extensions that were added later.
> > >
> > > Real hardware is constrained in not being able to invent arbitrary
> > > new features on chip. Virtual machines  are not constrained, so
> > > I don't think the inability of hardware todo this, is an especially
> > > strong reason to limit software emulation.

I think exposing flexibility in vendor CPUs just creates confusion.

As a user if I start QEMU with "-cpu company-cpu" then I am expecting
to get an emulation of company-cpu.

> > >
> > > What I don't like about this, is that (IIUC) the '$profile=on' option
> > > now has different semantics depending on what CPU it is used with.
> > >
> > > ie  using it with a vendor CPU,   $profile=on  becomes an assertion
> > > that the vendor CPU contains all the features needed to satisfy
> > > $profile. It won't enable/disable anything, just check it is present.
> > >
> > > With a non-vendor CPU, using $profile=on becomes a mechanism to force
> > > enable all the features needed to satisfy $profile, there is no
> > > mechanism to just check for presence.
> > >
> > > Having two different semantics for the same syntax is generally considered
> > > bad design practice.
> > >
> > > This points towards supporting a tri-state, not boolean. $profile=check
> > > for validation only, and $profile=on for force enablement.
> >
> > This would leave us with:
> >
> > - $profile=off => disable all extensions. Let users hit themselves in the 
> > foot if they
> > don't enable any other extensions. Note that disabling a profile and 
> > enabling extensions
> > on top of it is very sensitive to left-to-right ordering, so it would be 
> > good to have
> > a way to enforce this ordering somehow (feature groups always first);
>
> It is also order sensitive if 2 profiles have overlap in the
> extensions they represent. So might also require an ordering
> of profiles themselves to be defined if you permit multiple
> profiles.
>
> If we dont want to think about this immediately that, then
> we should make $profile=off into a fatal error rather than
> silently ignoring it

I think that makes sense.

I think we can be pretty strict on profiles options. To me it seems
reasonable to say a user can enable **one** profile. Once that profile
is enabled they get all of those extensions.

If possible/simple we can then allow them to manually enable and/or
disable extensions on top of that. I don't see any use in allowing
users to turn profiles "off" though. I'm not even clear what that
means.

Alistair

>
> > - $profile=on => only valid for generic CPUs;
> >
> > - $profile=check -> valid for all CPUs, would only check if the CPU 
> > implements the profile.
> >
> >
> > I think this is fine. Drew, care to weight in?
>
>
> With regards,
> Daniel
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