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

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Message-ID: <CAFEAcA-L7sQfK6MNt1ZbZqUMk+TJor=uD3Jj-Pc6Vy9j9JHhYQ@mail.gmail.com>
Date: Wed, 18 Sep 2024 14:12:48 +0100
From: Peter Maydell <peter.maydell@...aro.org>
To: Heinrich Schuchardt <heinrich.schuchardt@...onical.com>
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 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