Re: [PATCH] slow_map: minor improvements to ROM BAR handling

Michael S. Tsirkin
2009-12-22T16:07:49Z

On Tue, Dec 22, 2009 at 05:00:52PM +0100, Alexander Graf wrote:
> Avi Kivity wrote:
> > On 12/22/2009 05:41 PM, Alexander Graf wrote:
> >>
> >>> We could certainly extend emulate.c to fetch instruction bytes from
> >>> userspace. It uses ->read_std() now, so we'd need to switch to
> >>> ->read_emulated() and add appropriate buffering.
> >>>
> >> I thought the policy on emulate.c was to not have a full instruction
> >> emulator but only emulate instructions that do PT modifications or MMIO
> >> access?
> >>
> >
> > It's not a policy, just laziness. With emulate_invalid_guest_state=1
> > we need many more instructions. Of course I don't want to add
> > instructions just for the sake of it, since they will be untested.
> >
> > I'd much prefer not to run from mmio if possible - just pointing out
> > it's doable.
>
> Right...
>
> >> emulator is _really_ small. It only does a few MMU specific
> >> instructions, a couple of privileged ones and MMIO accessing ones.
> >>
> > Btw, we're in the same situation with PowerPC here. The instruction
> >
> > Plus, you have a fixed length instruction length, likely more regular
> > too. I imagine powerpc is load/store, so you don't have to emulate a
> > zillion ALU instructions?
>
> Well, it's certainly doable (and easier than on x86). But I'm on the
> same position as you on the x86 side. Why increase the emulator size at
> least 10 times if we don't have to?
>
> Either way, people will report bugs when / if they actually start
> executing code off MMIO. So let's not care too much about it for now.
> Just make sure the read-only check is in.
>
> Alex

So I think all we need is this on top?

diff --git a/hw/device-assignment.c b/hw/device-assignment.c
index 066fdb6..0c3c8f4 100644
--- a/hw/device-assignment.c
+++ b/hw/device-assignment.c
@@ -233,7 +233,8 @@ static void assigned_dev_iomem_map_slow(PCIDevice *pci_dev, int region_num,
int m;

DEBUG("slow map\n");
- m = cpu_register_io_memory(slow_bar_read, slow_bar_write, region);
+ m = cpu_register_io_memory(slow_bar_read, region_num == PCI_ROM_SLOT ?
+ NULL : slow_bar_write, region);
cpu_register_physical_memory(e_phys, e_size, m);

/* MSI-X MMIO page */