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Re: [Qemu-devel] [PATCH v6 26/26] RISC-V: Workaround for critical mstatu
From: |
Michael Clark |
Subject: |
Re: [Qemu-devel] [PATCH v6 26/26] RISC-V: Workaround for critical mstatus.FS MTTCG bug |
Date: |
Sat, 24 Mar 2018 12:17:33 -0700 |
On Sat, Mar 24, 2018 at 11:13 AM, Michael Clark <address@hidden> wrote:
> This change is a workaround for a bug where mstatus.FS
> is not correctly reporting dirty when MTTCG and SMP are
> enabled which results in the floating point register file
> not being saved during context switches. This a critical
> bug for RISC-V in QEMU as it results in floating point
> register file corruption when running SMP Linux in the
> RISC-V 'virt' machine.
>
> This workaround will return dirty if mstatus.FS is
> switched from off to initial or clean. We have checked
> the specification and it is legal for an implementation
> to return either off, or dirty, if set to initial or clean.
>
> This workaround will result in unnecessary floating point
> save restore. When mstatus.FS is off, floating point
> instruction trap to indicate the process is using the FPU.
> The OS can then save floating-point state of the previous
> process using the FPU and set mstatus.FS to initial or
> clean. With this workaround, mstatus.FS will always return
> dirty if set to a non-zero value, indicating floating point
> save restore is necessary, versus misreporting mstatus.FS
> resulting in floating point register file corruption.
>
> Cc: Palmer Dabbelt <address@hidden>
> Cc: Sagar Karandikar <address@hidden>
> Cc: Bastian Koppelmann <address@hidden>
> Cc: Richard W.M. Jones <address@hidden>
> Cc: Peter Maydell <address@hidden>
> Signed-off-by: Michael Clark <address@hidden>
> ---
> target/riscv/op_helper.c | 19 +++++++++++++++++--
> 1 file changed, 17 insertions(+), 2 deletions(-)
>
> diff --git a/target/riscv/op_helper.c b/target/riscv/op_helper.c
> index 1fdde90..d345688 100644
> --- a/target/riscv/op_helper.c
> +++ b/target/riscv/op_helper.c
> @@ -144,8 +144,23 @@ void csr_write_helper(CPURISCVState *env,
> target_ulong val_to_write,
> }
>
> mstatus = (mstatus & ~mask) | (val_to_write & mask);
> - int dirty = (mstatus & MSTATUS_FS) == MSTATUS_FS;
> - dirty |= (mstatus & MSTATUS_XS) == MSTATUS_XS;
> +
> + /* Note: this is a workaround for an issue where mstatus.FS
> + does not report dirty when SMP and MTTCG is enabled. This
> + workaround is technically compliant with the RISC-V Privileged
> + specification as it is legal to return only off, or dirty,
> + however this may cause unnecessary saves of floating point
> state.
> + Without this workaround, floating point state is not saved and
> + restored coorectly when SMP and MTTCG is enabled, */
>
typo in "correctly". I will fix this...
> + if (qemu_tcg_mttcg_enabled()) {
> + /* FP is always dirty or off */
> + if (mstatus & MSTATUS_FS) {
> + mstatus |= MSTATUS_FS;
> + }
> + }
> +
> + int dirty = ((mstatus & MSTATUS_FS) == MSTATUS_FS) |
> + ((mstatus & MSTATUS_XS) == MSTATUS_XS);
> mstatus = set_field(mstatus, MSTATUS_SD, dirty);
> env->mstatus = mstatus;
> break;
> --
> 2.7.0
>
>
- Re: [Qemu-devel] [PATCH v6 06/26] RISC-V: Mark ROM read-only after copying in code, (continued)
[Qemu-devel] [PATCH v6 19/26] RISC-V: vectored traps are optional, Michael Clark, 2018/03/24
[Qemu-devel] [PATCH v6 10/26] RISC-V: Improve page table walker spec compliance, Michael Clark, 2018/03/24
[Qemu-devel] [PATCH v6 08/26] RISC-V: Make sure rom has space for fdt, Michael Clark, 2018/03/24
[Qemu-devel] [PATCH v6 26/26] RISC-V: Workaround for critical mstatus.FS MTTCG bug, Michael Clark, 2018/03/24
[Qemu-devel] [PATCH v6 20/26] RISC-V: No traps on writes to misa, minstret, mcycle, Michael Clark, 2018/03/24
[Qemu-devel] [PATCH v6 21/26] RISC-V: Remove support for adhoc X_COP interrupt, Michael Clark, 2018/03/24
Re: [Qemu-devel] [PATCH v6 00/26] RISC-V: Fixes and cleanups for QEMU 2.12, Peter Maydell, 2018/03/25
Re: [Qemu-devel] [patches] Re: [PATCH v6 00/26] RISC-V: Fixes and cleanups for QEMU 2.12, Peter Maydell, 2018/03/27
Re: [Qemu-devel] [patches] Re: [PATCH v6 00/26] RISC-V: Fixes and cleanups for QEMU 2.12, Michael Clark, 2018/03/27