linux-kernel - Re: [PATCH 1/1] target/riscv: enable floating point unit

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Message-ID: <ca16bb60-9745-478d-afba-1330b385027e@canonical.com>
Date: Wed, 18 Sep 2024 18:27:14 +0200
From: Heinrich Schuchardt <heinrich.schuchardt@...onical.com>
To: Peter Maydell <peter.maydell@...aro.org>
Cc: Palmer Dabbelt <palmer@...belt.com>,
 Alistair Francis <alistair.francis@....com>, Bin Meng <bmeng.cn@...il.com>,
 Weiwei Li <liwei1518@...il.com>,
 Daniel Henrique Barboza <dbarboza@...tanamicro.com>,
 Liu Zhiwei <zhiwei_liu@...ux.alibaba.com>, qemu-riscv@...gnu.org,
 qemu-devel@...gnu.org, Anup Patel <anup@...infault.org>,
 Atish Patra <atishp@...shpatra.org>, Paul Walmsley
 <paul.walmsley@...ive.com>, Albert Ou <aou@...s.berkeley.edu>,
 kvm@...r.kernel.org, kvm-riscv@...ts.infradead.org,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
 Andrew Jones <ajones@...tanamicro.com>
Subject: Re: [PATCH 1/1] target/riscv: enable floating point unit

On 18.09.24 17:32, Peter Maydell wrote:
> On Wed, 18 Sept 2024 at 14:49, Heinrich Schuchardt
> <heinrich.schuchardt@...onical.com> wrote:
>>
>> On 18.09.24 15:12, Peter Maydell wrote:
>>> On Wed, 18 Sept 2024 at 14:06, Heinrich Schuchardt
>>> <heinrich.schuchardt@...onical.com> wrote:
>>>> Thanks Peter for looking into this.
>>>>
>>>> QEMU's cpu_synchronize_all_post_init() and
>>>> do_kvm_cpu_synchronize_post_reset() both end up in
>>>> kvm_arch_put_registers() and that is long after Linux
>>>> kvm_arch_vcpu_create() has been setting some FPU state. See the output
>>>> below.
>>>>
>>>> kvm_arch_put_registers() copies the CSRs by calling
>>>> kvm_riscv_put_regs_csr(). Here we can find:
>>>>
>>>>        KVM_RISCV_SET_CSR(cs, env, sstatus, env->mstatus);
>>>>
>>>> This call enables or disables the FPU according to the value of
>>>> env->mstatus.
>>>>
>>>> So we need to set the desired state of the floating point unit in QEMU.
>>>> And this is what the current patch does both for TCG and KVM.
>>>
>>> If it does this for both TCG and KVM then I don't understand
>>> this bit from the commit message:
>>>
>>> # Without this patch EDK II with TLS enabled crashes when hitting the first
>>> # floating point instruction while running QEMU with --accel kvm and runs
>>> # fine with --accel tcg.
>>>
>>> Shouldn't this guest crash the same way with both KVM and TCG without
>>> this patch, because the FPU state is the same for both?
> 
>> By default `qemu-system-riscv64 --accel tcg` runs OpenSBI as firmware
>> which enables the FPU.
>>
>> If you would choose a different SBI implementation which does not enable
>> the FPU you could experience the same crash.
> 
> Ah, so KVM vs TCG is a red herring and it's actually "some guest
> firmware doesn't enable the FPU itself, and if you run that then it will
> fall over, whether you do it in KVM or TCG" ? That makes more sense.
> 
> I don't have an opinion on whether you want to do that or not,
> not knowing what the riscv architecture mandates. (On Arm this
> would be fairly clearly "the guest software is broken and
> should be fixed", but that's because the Arm architecture
> says you can't assume the FPU is enabled from reset.)
> 
> I do think the commit message could use clarification to
> explain this.
> 
> thanks
> -- PMM

I have not found a specification defining what the status of the FPU 
should be when M-Mode is stared and when moving from M-Mode to S-Mode.

OpenSBI (which is the dominating M-Mode firmware and invoked by default 
in TCG mode) enables the FPU before jumping to S-Mode. KVM should to the 
same for consistency.

Best regards

Heinrich