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

[Daniel P . Berrangé]

On Fri, Sep 29, 2023 at 10:26:08AM -0300, Daniel Henrique Barboza wrote:
> 
> 
> On 9/29/23 09:52, Daniel P. Berrangé 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.
> > > > 
> > > > 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 don't mind handling it right now, I just don't know how hehe
> 
> I'll re-read the thread you sent earlier and see if there's something I 
> missed. I got the
> impression that we need your qom patch first.

My patch is dropped as it was considered too gross. Kevin provided
a new impl for ARRAY properties which eliminates the ordering problem.

IOW, QemuOpts iteration order will remain undefined in general.

With regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|