Re: [Qemu-devel] [PATCH v4 3/7] target/riscv: Create function to test if FP is enabled

[Aurelien Jarno]

On 2020-01-05 17:36, Aurelien Jarno wrote:
> > diff --git a/target/riscv/csr.c b/target/riscv/csr.c
> > index e0d4586760..2789215b5e 100644
> > --- a/target/riscv/csr.c
> > +++ b/target/riscv/csr.c
> 
> [ snip ]
> 
> > @@ -307,6 +307,7 @@ static int write_mstatus(CPURISCVState *env, int csrno, 
> > target_ulong val)
> >  {
> >      target_ulong mstatus = env->mstatus;
> >      target_ulong mask = 0;
> > +    int dirty;
> >  
> >      /* flush tlb on mstatus fields that affect VM */
> >      if (env->priv_ver <= PRIV_VERSION_1_09_1) {
> > @@ -340,8 +341,9 @@ static int write_mstatus(CPURISCVState *env, int csrno, 
> > target_ulong val)
> >  
> >      mstatus = (mstatus & ~mask) | (val & mask);
> >  
> > -    int dirty = ((mstatus & MSTATUS_FS) == MSTATUS_FS) |
> > -                ((mstatus & MSTATUS_XS) == MSTATUS_XS);
> > +    dirty = (riscv_cpu_fp_enabled(env) &&
> > +             ((mstatus & MSTATUS_FS) == MSTATUS_FS)) |
> > +            ((mstatus & MSTATUS_XS) == MSTATUS_XS);
> >      mstatus = set_field(mstatus, MSTATUS_SD, dirty);
> >      env->mstatus = mstatus;
> 
> This patch, and more precisely the above two hunks broke
> qemu-system-riscv64. More precisely, when running a Debian sid system
> inside QEMU, sshd hangs during key exchange.

The problem is that at this stage, mstatus != env->status. Prior to that
patch, dirty was computed exclusively on the new mstatus status, after
the update by val. With this patch, riscv_cpu_fp_enabled() refers to the
old value of mstatus. Therefore when FS is changed from "Off" (FS = 00)
to "Dirty" (FS == 11), the SD bit is not set.

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
address@hidden                 http://www.aurel32.net