Re: [Qemu-devel] [PATCH v6 26/26] RISC-V: Workaround for critical mstatus.FS MTTCG bug

[Richard W.M. Jones]

On Sat, Mar 24, 2018 at 11:13:40AM -0700, Michael Clark 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>

I tested this by running qemu from git with and without this patch,
both times compiling and running the “sched.c” test program:

http://oirase.annexia.org/tmp/sched.c

In my tests it fixes the problem, and therefore:

Tested-by: Richard W.M. Jones <address@hidden>

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-p2v converts physical machines to virtual machines.  Boot with a
live CD or over the network (PXE) and turn machines into KVM guests.
http://libguestfs.org/virt-v2v