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

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Message-ID: <f1e41b95-c499-4e06-91cb-006dcd9d29e6@canonical.com>
Date: Wed, 18 Sep 2024 15:49:39 +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 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?
> 
> -- PMM

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.

Best regards

Heinrich