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

[Daniel P . Berrangé]

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.

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.


With regards,
Daniel
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