Re: [Qemu-devel] [PATCH 2/5] acpi_piix4: Only allow writes to PCI hotplug eject register

[Michael S. Tsirkin]

On Thu, Apr 05, 2012 at 12:40:00PM +0300, Gleb Natapov wrote:
> On Thu, Apr 05, 2012 at 12:37:01PM +0300, Michael S. Tsirkin wrote:
> > On Thu, Apr 05, 2012 at 12:12:37PM +0300, Gleb Natapov wrote:
> > > On Thu, Apr 05, 2012 at 12:04:44PM +0300, Michael S. Tsirkin wrote:
> > > > On Thu, Apr 05, 2012 at 10:21:18AM +0200, Igor Mammedov wrote:
> > > > > On 04/05/2012 07:51 AM, Alex Williamson wrote:
> > > > > >This is never read.  We can also derive bus from the write handler,
> > > > > >making this more inline with the other callbacks.  Note that
> > > > > >pciej_write was actually called with (PCIBus *)dev->bus, which is
> > > > > >cast as a void* allowing us to pretend it's a BusState*.  Fix this
> > > > > >so we don't depend on the BusState location within PCIBus.
> > > > > >
> > > > > >Signed-off-by: Alex Williamson<address@hidden>
> > > > > >---
> > > > > >
> > > > > >  docs/specs/acpi_pci_hotplug.txt |    2 +-
> > > > > >  hw/acpi_piix4.c                 |   14 ++++----------
> > > > > >  2 files changed, 5 insertions(+), 11 deletions(-)
> > > > > >
> > > > > >diff --git a/docs/specs/acpi_pci_hotplug.txt 
> > > > > >b/docs/specs/acpi_pci_hotplug.txt
> > > > > >index 1e2c8a2..1e61d19 100644
> > > > > >--- a/docs/specs/acpi_pci_hotplug.txt
> > > > > >+++ b/docs/specs/acpi_pci_hotplug.txt
> > > > > >@@ -28,7 +28,7 @@ PCI device eject (IO port 0xae08-0xae0b, 4-byte 
> > > > > >access):
> > > > > >  ----------------------------------------
> > > > > >
> > > > > >  Used by ACPI BIOS _EJ0 method to request device removal. One bit 
> > > > > > per slot.
> > > > > >-Reads return 0.
> > > > > >+Read-only.
> > > > > Write-only perhaps?
> > > > 
> > > > Yes, let's also specify what happens in practice.
> > > No we shouldn't.
> > > 
> > > > I think it is 'Guest should never read this register, in practice
> > > > 0 is returned'.
> > > > 
> > > In practice kitten die for each read. Unspecified behaviour is
> > > unspecified.
> > 
> > 
> > Why, what are you worried about? I just want to document what we do.
> > 
> You are making undefined behaviour to be defined one.
> 
> > The reason I want to specify behaviour on read is because down
> > the road we might want to return something here. Our lives
> > will be easier if we have a document which we can read
> > and figure out what old qemu did.
> > 
> You can do all that only if behaviour is undefined. If it is defined you
> can't change it.

We are doing just that constantly, just be careful.
Documenting what happens will make it easier.

> Our lives will be easier if we will leave undefined
> behaviour undefined.

So yes live it undefined for guests but document what happens
for ourselves. Let's make it explicit, say
'current implementation returns 0 this can
 change at any time without notice.'

I want to go further. For up/down I would like to
document pas behaviour as well
'past implementations made the registers
read-write, writing there would clobber
outstanding hotplug requests.
current implementation makes the register read-only,
writes are discarded.
'

Just undefined is vague. If someone bothered
documenting the current undefined behavour
with registers being writeable instead of
undefined, then people would detect this
as a bug and this would have been fixed.


> --
>                       Gleb.