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

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Message-ID: <20240918-4e2df3f0cabdb8002d7315d9@orel>
Date: Wed, 18 Sep 2024 15:56:58 +0200
From: Andrew Jones <ajones@...tanamicro.com>
To: Heinrich Schuchardt <heinrich.schuchardt@...onical.com>
Cc: Peter Maydell <peter.maydell@...aro.org>, 
	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
Subject: Re: [PATCH 1/1] target/riscv: enable floating point unit

On Wed, Sep 18, 2024 at 03:49:39PM GMT, Heinrich Schuchardt 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?
> > 
> > -- 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.
> 

Thanks Heinrich, I had also forgotten that distinction. So the last
question is whether or not we want to reset mstatus.FS to 1 instead of 3,
as is done in this patch.

Thanks,
drew