From: "Daniel P. Berrangé" <berrange@redhat.com>
To: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Cc: qemu-devel@nongnu.org, qemu-riscv@nongnu.org,
alistair.francis@wdc.com, bmeng@tinylab.org,
liweiwei@iscas.ac.cn, zhiwei_liu@linux.alibaba.com,
palmer@rivosinc.com, Andrew Jones <ajones@ventanamicro.com>
Subject: Re: [PATCH 0/6] riscv: RVA22U64 profile support
Date: Fri, 29 Sep 2023 12:55:39 +0100 [thread overview]
Message-ID: <ZRa7O67ZTukOq5GL@redhat.com> (raw)
In-Reply-To: <e5342929-506a-ce75-34fa-204ad0970ee2@ventanamicro.com>
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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next prev parent reply other threads:[~2023-09-29 11:56 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-09-26 19:49 [PATCH 0/6] riscv: RVA22U64 profile support Daniel Henrique Barboza
2023-09-26 19:49 ` [PATCH 1/6] target/riscv/cpu.c: add zicntr extension flag Daniel Henrique Barboza
2023-09-26 19:49 ` [PATCH 2/6] target/riscv/cpu.c: add zihpm " Daniel Henrique Barboza
2023-09-26 19:49 ` [PATCH 3/6] target/riscv: add rva22u64 profile definition Daniel Henrique Barboza
2023-09-26 19:49 ` [PATCH 4/6] target/riscv/tcg: implement rva22u64 profile Daniel Henrique Barboza
2023-09-26 19:49 ` [PATCH 5/6] target/riscv/tcg-cpu.c: enable profile support for vendor CPUs Daniel Henrique Barboza
2023-09-26 19:49 ` [PATCH 6/6] target/riscv/kvm: add 'rva22u64' flag as unavailable Daniel Henrique Barboza
2023-09-29 10:10 ` [PATCH 0/6] riscv: RVA22U64 profile support Andrea Bolognani
2023-09-29 10:46 ` Daniel P. Berrangé
2023-09-29 11:29 ` Daniel Henrique Barboza
2023-09-29 11:55 ` Daniel P. Berrangé [this message]
2023-09-29 12:49 ` Daniel Henrique Barboza
2023-09-29 12:52 ` Daniel P. Berrangé
2023-09-29 13:26 ` Daniel Henrique Barboza
2023-09-29 13:32 ` Daniel P. Berrangé
2023-10-09 2:32 ` Alistair Francis
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