From: Jose Martins <josemartins90@gmail.com>
To: Alistair Francis <alistair23@gmail.com>
Cc: "open list:RISC-V TCG CPUs" <qemu-riscv@nongnu.org>,
Sagar Karandikar <sagark@eecs.berkeley.edu>,
Bastian Koppelmann <kbastian@mail.uni-paderborn.de>,
"qemu-devel@nongnu.org Developers" <qemu-devel@nongnu.org>,
Alistair Francis <Alistair.Francis@wdc.com>,
Palmer Dabbelt <palmer@dabbelt.com>
Subject: Re: [PATCH] target/riscv: fix wfi exception behavior
Date: Fri, 16 Apr 2021 12:34:37 +0100 [thread overview]
Message-ID: <CAC41xo1K6_K7YXn2E6inRf0spkbOAGQNy2eA0s6TWXV7V_aYhg@mail.gmail.com> (raw)
In-Reply-To: <CAKmqyKPpjRfowvYjofny8JNwnDUZkyH4jnU6mvVxaWNJuEgsqA@mail.gmail.com>
> > - if ((env->priv == PRV_S &&
> > - get_field(env->mstatus, MSTATUS_TW)) ||
> > - riscv_cpu_virt_enabled(env)) {
> > + if ((!riscv_cpu_virt_enabled(env) && env->priv == PRV_U) ||
>
> > + (env->priv == PRV_S && get_field(env->mstatus, MSTATUS_TW))) {
>
> Shouldn't we check here that we aren't virtualised?
>
In section 5.4, the spec states that mstatus.tw has effect regardless
of virtualization mode: "The TW field affects execution in all modes
except M-mode.". I interpret "all modes" as being all supervisor modes
since section 3.1.6.5 states that "When S-mode is implemented, then
executing WFI in U-mode causes an illegal instruction exception" and
later chapter 5 says that a virtual instruction exception should be
generated when "in VU-mode, attempts to execute WFI (...)" regardless
of the state of any status bit. Plus, it should be an illegal
instruction exception and not a virtual instruction exception even in
VS-mode when mstatus.tw = 1 because the spec also states only "When
VTW=1 *(and assuming mstatus.TW=0)*, an attempt in VS-mode to execute
WFI raises a virtual instruction exception".
But just now I realized the patch is assuming S-mode is present and
not taking into account M/U only harts. If this is the case TW should
affect U-mode WFIs. I will fix this and submit a new version of the
patch.
José
prev parent reply other threads:[~2021-04-16 11:36 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-10 19:40 [PATCH] target/riscv: fix wfi exception behavior Jose Martins
2021-04-16 4:55 ` Alistair Francis
2021-04-16 11:34 ` Jose Martins [this message]
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