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Re: [PATCH v3 3/6] target/riscv/tcg: add user flag for profile support
From: |
Andrew Jones |
Subject: |
Re: [PATCH v3 3/6] target/riscv/tcg: add user flag for profile support |
Date: |
Mon, 23 Oct 2023 19:35:16 +0200 |
On Mon, Oct 23, 2023 at 02:00:00PM -0300, Daniel Henrique Barboza wrote:
>
>
> On 10/23/23 05:16, Andrew Jones wrote:
> > On Fri, Oct 20, 2023 at 07:39:48PM -0300, Daniel Henrique Barboza wrote:
> > > The TCG emulation implements all the extensions described in the
> > > RVA22U64 profile, both mandatory and optional. The mandatory extensions
> > > will be enabled via the profile flag. We'll leave the optional
> > > extensions to be enabled by hand.
> > >
> > > Given that this is the first profile we're implementing in TCG we'll
> > > need some ground work first:
> > >
> > > - all profiles declared in riscv_profiles[] will be exposed to users.
> > > TCG is the main accelerator we're considering when adding profile
> > > support in QEMU, so for now it's safe to assume that all profiles in
> > > riscv_profiles[] will be relevant to TCG;
> > >
> > > - we'll not support user profile settings for vendor CPUs. The flags
> > > will still be exposed but users won't be able to change them. The idea
> > > is that vendor CPUs in the future can enable profiles internally in
> > > their cpu_init() functions, showing to the external world that the CPU
> > > supports a certain profile. But users won't be able to enable/disable
> > > it;
> > >
> > > - Setting a profile to 'true' means 'enable all mandatory extensions of
> > > this profile, setting it to 'false' means disabling all its mandatory
> > > extensions. Disabling a profile is discouraged for regular use and will
> > > issue an user warning. User choices for individual extensions will take
> > > precedence, i.e. enabling a profile will not enable extensions that the
> > > user set to 'false', and vice-versa. This will make us independent of
> > > left-to-right ordering in the QEMU command line, i.e. the following QEMU
> > > command lines:
> > >
> > > -cpu rv64,zicbom=false,rva22u64=true,Zifencei=false
> > > -cpu rv64,zicbom=false,Zifencei=false,rva22u64=true
> > > -cpu rv64,rva22u64=true,zicbom=false,Zifencei=false
> > >
> > > They mean the same thing: "enable all mandatory extensions of the
> > > rva22u64 profile while keeping zicbom and Zifencei disabled".
> >
> > Hmm, I'm not sure I agree with special-casing profiles like this. I think
> > the left-to-right processing should be consistent for all. I'm also not
> > sure we should always warn when disabling a profile. For example, if a
> > user does
> >
> > -cpu rv64,rva22u64=true,rva22u64=false
> >
> > then they'll get a warning, even though all they're doing is restoring the
> > cpu model. While that looks like an odd thing to do, a script may be
> > adding the rva22u64=true and the rva22u64=false is the user input which
> > undoes what the script did.
>
> QEMU options do not work with a "the user enabled then disabled the same
> option,
> thus it'll count as nothing happened" logic. The last instance of the option
> will
> overwrite all previous instances. In the example you mentioned above the user
> would
> disable all mandatory extensions of rva22u64 in the CPU, doesn't matter if the
> same profile was enabled beforehand.
Yup, I'm aware, but I keep thinking that we'll only be using profiles with
a base cpu type. If you start with nothing (a base) and then add a profile
and take the same one away, you shouldn't be taking away anything else. I
agree that if you use a profile on some cpu type that already enabled a
bunch of stuff itself, then disabling a profile would potentially remove
some of those too, but mixing cpu types that have their own extensions and
profiles seems like a great way to confuse oneself as to what extensions
will be present. IOW, we should be adding a base cpu type at the same
time we're adding these profiles.
>
> Sure, the can put code in place to make this happen, but then this would make
> profiles act different than regular extensions. "-cpu rv64,zicbom=true -cpu
> rv64,zicbom=false"
> will disable zicbom, it will not preserve the original 'zicbom' rv64 default.
> If
> we're going to keep left-to-right ordering consistent, this behavior should
> also
> be consistent as well.
It will be consistent if we always override whatever was on the left with
what's on the right, which means with
-cpu rv64,rva22u64=true -cpu rv64,zicbom=false
zicbom will be disabled, but with
-cpu rv64,zicbom=false -cpu rv64,rva22u64=true
it will be enabled. The same goes if the properties are given to the same
-cpu parameter.
>
>
> As for warnings, I agree that we'll throw warnings even when nothing of
> notice happened.
> For example:
>
> -cpu rv64,rva22u64=false -cpu rv64,rva22u64=true
>
> This will throw a warning even though the user ended up enabling the extension
> in the end.
>
>
> We can fix it by postponing warnings to realize().
>
>
> >
> > As far as warnings go, it'd be nice to warn when mandatory profile
> > extensions are disabled from an enabled profile. Doing that might be
> > useful for debug, but users which do it without being aware they're
> > "breaking" the profile may learn from that warning. Note, the warning
> > should only come when the profile is actually enabled and when the
> > extension would actually be disabled, i.e.
> >
> > -cpu rv64,rva22u64=true,c=off
> >
> > should warn
> >
> > -cpu rv64,c=off,rva22u64=true
> >
> > should not warn (rva22u64 overrides c=off since it's to the right)
> >
> > -cpu rv64,rva22u64=true,rva22u64=false,c=off
> >
> > should not warn (rva22u64 is not enabled)
>
> Ack for all the above.
>
> >
> > And,
> >
> > -cpu rv64,rva22u64=true,rva24u64=false
> >
> > should warn for each extension which is mandatory in both profiles.
>
> The way I'm imagining this happening is to cycle through all profiles during
> realize(),
> see which ones are enabled, and then warn if the user disabled their mandatory
> extensions. In this example we would warn for all rva22 mandatory extensions
> that were disabled because we disabled rva24, but we won't emit any warnings
> for
> rva24 mandatory extensions given that the profile is marked as disabled.
Yup, sounds good.
Thanks,
drew
- [PATCH v3 1/6] target/riscv: add rva22u64 profile definition, (continued)
- [PATCH v3 1/6] target/riscv: add rva22u64 profile definition, Daniel Henrique Barboza, 2023/10/20
- [PATCH v3 2/6] target/riscv/kvm: add 'rva22u64' flag as unavailable, Daniel Henrique Barboza, 2023/10/20
- [PATCH v3 4/6] target/riscv/tcg: add MISA user options hash, Daniel Henrique Barboza, 2023/10/20
- [PATCH v3 3/6] target/riscv/tcg: add user flag for profile support, Daniel Henrique Barboza, 2023/10/20
- Re: [PATCH v3 3/6] target/riscv/tcg: add user flag for profile support, Andrew Jones, 2023/10/23
- Re: [PATCH v3 3/6] target/riscv/tcg: add user flag for profile support, Daniel Henrique Barboza, 2023/10/23
- Re: [PATCH v3 3/6] target/riscv/tcg: add user flag for profile support,
Andrew Jones <=
- Re: [PATCH v3 3/6] target/riscv/tcg: add user flag for profile support, Daniel Henrique Barboza, 2023/10/23
- Re: [PATCH v3 3/6] target/riscv/tcg: add user flag for profile support, Andrea Bolognani, 2023/10/26
- Re: [PATCH v3 3/6] target/riscv/tcg: add user flag for profile support, Andrew Jones, 2023/10/26
- Re: [PATCH v3 3/6] target/riscv/tcg: add user flag for profile support, Andrea Bolognani, 2023/10/26
- Re: [PATCH v3 3/6] target/riscv/tcg: add user flag for profile support, Daniel Henrique Barboza, 2023/10/27
- Re: [PATCH v3 3/6] target/riscv/tcg: add user flag for profile support, Andrew Jones, 2023/10/28
Re: [PATCH v3 3/6] target/riscv/tcg: add user flag for profile support, LIU Zhiwei, 2023/10/25
[PATCH v3 6/6] target/riscv/tcg: handle profile MISA bits, Daniel Henrique Barboza, 2023/10/20
[PATCH v3 5/6] target/riscv/tcg: add riscv_cpu_write_misa_bit(), Daniel Henrique Barboza, 2023/10/20