"Michael S. Tsirkin" <> writes:
> On Fri, Oct 04, 2013 at 01:06:43PM +0930, Rusty Russell wrote:
>> "Michael S. Tsirkin" <> writes:
>> > On Tue, Sep 24, 2013 at 08:18:20PM +0930, Rusty Russell wrote:
>> >> I don't think qemu supports changing a disk underneath a device anyway?
>> >
>> > QEMU supports resizing I think.
>> >
>> >> And in practice, I'm not sure it would ever change fast enough to
>> >> trigger this race...
>> >
>> > Well it's easy to make guest very slow :)
>> >
>> >> So, should we try to fix it?
>> >> Rusty.
>> >
>> > Not really sure but I thought I'd put it on the table.
>>
>> OK, I got ambitious! How's this look?
>>
>> Cheers,
>> Rusty.
>>
>> commit 9aebe9c69be1461dd0110dfc61459a4ac7bda6f9
>> Author: Rusty Russell <>
>> Date: Fri Oct 4 13:01:35 2013 +0930
>>
>> Configuration (read) atomicity.
>>
>> Aka issue VIRTIO-35.
>>
>> This is solved per transport:
>> 1) PCI: use the 8 bit reserved field.
>> Assume that if you really change that fast, you'll do it lazily on
>> config space read.
>
> I guess it's possible ... but isn't it easier to just make it a 32 bit field
> than add this lazy update logic to devices?
I was trying to keep it tight. I think the lazy update logic is
pretty trivial:
bool config_changed;
....
on_config_generation_read()
{
if (config_changed) {
gen++;
config_changed = false;
}
return gen;
}
>> 2) MMIO: use offset 0x18.
>> Qemu returns 0 when they read here anyway, so it's almost 100%
>> backwards compatible (legacy drivers should still multi-read
>> for any transport).
>> 3) CCW: no transport changes.
>> They always read/write the entire thing. This just shows that
>> Cornelia is smarter than I am.
>>
>> Signed-off-by: Rusty Russell <>
>>
>> diff --git a/virtio-v1.0-wd01-part1-specification.txt b/virtio-v1.0-wd01-part1-specification.txt
>> index b5a5835..2945db4 100644
>> --- a/virtio-v1.0-wd01-part1-specification.txt
>> +++ b/virtio-v1.0-wd01-part1-specification.txt
>> @@ -201,11 +201,36 @@ Interface' in the section title.
>> -------------------------
>>
>> Configuration space is generally used for rarely-changing or
>> -initialization-time parameters.
>> +initialization-time parameters. Drivers must not assume reads from
>> +fields greater than 32 bits wide are atomic, nor or reads from
>> +multiple fields.
>> +
>> +Each transport provides a generation count for the configuration
>> +space, which must change whenever there is a possibility that two
>
> MUST?
>
>> +accesses to the configuration space can see different versions of that
>> +space.
>> +
>> +Thus drivers should read configuration space fields like so:
>
> SHOULD?
The convention is to use lower case for OASIS. It says in the template
preamble. I guess it's bold in more sophisticated formats.
>> +
>> + u32 before, after;
>> + do {
>> + before = get_config_generation(device);
>> + // read config entry/entries.
>> + after = get_config_generation(device);
>> + } while (after != before);
>
> BTW this implies reads must never have side
> effects (PCI ISR is the only exception, isn't it?).
Yes. That would be weird...
Cheers,
Rusty.
