Re: [Qemu-devel] [PATCH] Pass the drive's readonly attribute to the guest OS

[Gleb Natapov]

On Thu, Oct 15, 2009 at 11:55:20AM +0200, Kevin Wolf wrote:
> Am 15.10.2009 11:54, schrieb Gleb Natapov:
> > On Thu, Oct 15, 2009 at 11:50:39AM +0200, Kevin Wolf wrote:
> >> Am 15.10.2009 11:43, schrieb Gleb Natapov:
> >>> On Thu, Oct 15, 2009 at 11:36:59AM +0200, Kevin Wolf wrote:
> >>>> Am 14.10.2009 18:40, schrieb Naphtali Sprei:
> >>>>> Naphtali Sprei wrote:
> >>>>> <snip>
> >>>>>> I'm planning to investigate where qemu should check the read only 
> >>>>>> attribute before exeuting any write command
> >>>>>> to drives, would be sent in a different patch.
> >>>>>
> >>>>> revisiting it, if guest OS knows it's a read only device and tries to 
> >>>>> modify it, anyhow, we don't really care about error reporting,
> >>>>> as long as qemu doesn't crash (or modify the drive).
> >>>>
> >>>> If the right response to a write on a read-only device is defined in the
> >>>> specification (and it most probably is), we should still give the right
> >>>> response, even though the OS is doing something wrong.
> >>>>
> >>> And since our response to write error may be pausing a VM we shouldn't
> >>> allow this to be triggered by a guest OS.
> >>
> >> I thought we only pause the VM if we get an host IO error? But if you do
> >> want to stop it for all errors, you shouldn't start suppressing errors
> >> so that it doesn't stop.
> >>
> > We pause only on host IO errors, but if we open underlying file as
> > read only (do we?) and try to write into it we will get an IO error
> > in the host.
> 
> No, we'll return an error before a write request to the host is even issued.
> 
Who is "we"? If "we" == "bdrv_write()/dma_bdrv_write()" then it's all the same.
Upper layers don't actually care why block driver failed.

--
                        Gleb.