Re: [Qemu-devel] [PATCH] cpu_physical_memory_write_rom() needs to do TB invalidates

[David Gibson]

On Wed, Aug 22, 2012 at 08:02:11AM +0200, Alexander Graf wrote:
> 
> On 22.08.2012, at 07:57, David Gibson wrote:
> 
> > On Wed, Aug 22, 2012 at 07:55:31AM +0200, Alexander Graf wrote:
> >> 
> >> On 22.08.2012, at 06:59, David Gibson wrote:
> >> 
> >>> cpu_physical_memory_write_rom(), despite the name, can also be used to
> >>> write images into RAM - and will often be used that way if the machine
> >>> uses load_image_targphys() into RAM addresses.
> >>> 
> >>> However, cpu_physical_memory_write_rom(), unlike cpu_physical_memory_rw()
> >>> does invalidate any cached TBs which might be affected by the region
> >>> written.
> >>> 
> >>> This was breaking reset (under full emu) on the pseries machine - we 
> >>> loaded
> >>> our firmware image into RAM, and while executing it rewrite the code at
> >>> the entry point (correctly causing a TB invalidate/refresh).  When we
> >>> reset the firmware image was reloaded, but the TB from the rewrite was
> >>> still active and caused us to get an illegal instruction trap.
> >>> 
> >>> This patch fixes the bug by duplicating the tb invalidate code from
> >>> cpu_physical_memory_rw() in cpu_physical_memory_write_rom().
> >>> 
> >>> Signed-off-by: David Gibson <address@hidden>
> >>> ---
> >>> exec.c |    7 +++++++
> >>> 1 file changed, 7 insertions(+)
> >>> 
> >>> diff --git a/exec.c b/exec.c
> >>> index 5834766..eff40d7 100644
> >>> --- a/exec.c
> >>> +++ b/exec.c
> >>> @@ -3523,6 +3523,13 @@ void 
> >>> cpu_physical_memory_write_rom(target_phys_addr_t addr,
> >>>            /* ROM/RAM case */
> >>>            ptr = qemu_get_ram_ptr(addr1);
> >>>            memcpy(ptr, buf, l);
> >>> +            if (!cpu_physical_memory_is_dirty(addr1)) {
> >>> +                /* invalidate code */
> >>> +                tb_invalidate_phys_page_range(addr1, addr1 + l, 0);
> >>> +                /* set dirty bit */
> >>> +                cpu_physical_memory_set_dirty_flags(
> >>> +                    addr1, (0xff & ~CODE_DIRTY_FLAG));
> >>> +            }
> >> 
> >> Can't we just call cpu_physical_memory_rw in the RAM case? The
> >> function only tries to not do MMIO accesses on ROM pages, right?
> > 
> > Maybe.  It's not clear at all to me what cases
> > cpu_physical_memory_write_rom() is supposed to be for, as opposed to
> > just using cpu_physical_memory_rw().
> 
> I can only guess, but the code looks to me as if it wants to be a
> nop on ROM pages, while basically doing cpu_physical_memory_rw for
> RAM pages. Usually in QEMU, every non-RAM page gets treated as MMIO
> which might eventually lead to machine checks.

Maybe.  Anthony, can you make a ruling on this, or tell me who can.  I
don't really care how I fix it, but it's definitely broken right now.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson