Re: [ros-diffs] 01/01: [NULL] Additions for the Null driver.

1) I thought certain MDL fast I/O routines can get called at DISPATCH_LEVEL
2) For perf reasons, causing a page-fault in the middle of a "fast I/O" is
probably a bad idea.

That being said -- these concerns seem silly since we are talking about the
"Null" driver which... is entirely PAGED_CODE and clearly neither #1 or #2
are actual issues.

In Windows, the fast I/o table is part of the DEVICE_OBJECT itself. Not
sure why our driver allocates a separate pool allocation to begin with.

Best regards,
Alex Ionescu

On Mon, Apr 23, 2018 at 1:30 AM, Ged Murphy <[email protected]>
wrote:

> They're pagable in NT6, I don't know whether that's the case in NT5.
>
> Change looks good to me, I'm not really sure why Alex used NNP?
> Perhaps our kernel does/did things slightly differently back when he wrote
> the Null driver?
>
> Ged.
>
> -----Original Message-----
> From: Ros-dev <[email protected]> On Behalf Of Thomas Faber
> Sent: Monday, 23 April 2018 07:49
> To: Hermès Bélusca-Maïto <[email protected]>
> Cc: [email protected]
> Subject: Re: [ros-dev] [ros-diffs] 01/01: [NULL] Additions for the Null
> driver.
>
> On 2018-04-22 22:23, Hermès Bélusca-Maïto wrote:
> > diff --git a/drivers/base/null/null.c b/drivers/base/null/null.c index
> > 610e886ddd..0d4ed541de 100644
> > --- a/drivers/base/null/null.c
> > +++ b/drivers/base/null/null.c
> > @@ -181,26 +199,16 @@ DriverEntry(IN PDRIVER_OBJECT DriverObject,
> >       DriverObject->MajorFunction[IRP_MJ_READ] = NullDispatch;
> >       DriverObject->MajorFunction[IRP_MJ_LOCK_CONTROL] = NullDispatch;
> >       DriverObject->MajorFunction[IRP_MJ_QUERY_INFORMATION] =
> > NullDispatch;
> > +    DriverObject->DriverUnload = NullUnload;
> >
> > -    /* Allocate the fast I/O dispatch table */
> > -    FastIoDispatch = ExAllocatePoolWithTag(NonPagedPool,
> > -                                           sizeof(FAST_IO_DISPATCH),
> > -                                           'llun');
> > -    if (!FastIoDispatch)
> > -    {
> > -        /* Failed, cleanup */
> > -        IoDeleteDevice(DeviceObject);
> > -        return STATUS_INSUFFICIENT_RESOURCES;
> > -    }
> > -
> > -    /* Initialize it */
> > -    RtlZeroMemory(FastIoDispatch, sizeof(FAST_IO_DISPATCH));
> > -    FastIoDispatch->SizeOfFastIoDispatch = sizeof(FAST_IO_DISPATCH);
> > +    /* Initialize the fast I/O dispatch table */
> > +    RtlZeroMemory(&FastIoDispatch, sizeof(FastIoDispatch));
> > +    FastIoDispatch.SizeOfFastIoDispatch = sizeof(FastIoDispatch);
> >
> >       /* Setup our pointers */
> > -    FastIoDispatch->FastIoRead = NullRead;
> > -    FastIoDispatch->FastIoWrite = NullWrite;
> > -    DriverObject->FastIoDispatch = FastIoDispatch;
> > +    FastIoDispatch.FastIoRead = NullRead;
> > +    FastIoDispatch.FastIoWrite = NullWrite;
> > +    DriverObject->FastIoDispatch = &FastIoDispatch;
>
>
> Are you sure FAST_IO_DISPATCH is allowed to be pageable? It seems to only
> be used at low IRQLs, so it seems logical. However I see it allocated
> nonpaged everywhere else and can't seem to find definitive documentation on
> the subject.
> (And yes, most filesystem drivers use a static structure, but those
>   drivers don't use MmPageEntireDriver)
>
> Thanks,
> Thomas
>
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