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

[Alex Ionescu]

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 <gedmurphy.maillists(a)gmail.com>
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 <ros-dev-bounces(a)reactos.org> On Behalf Of Thomas Faber
>  Sent: Monday, 23 April 2018 07:49
>  To: Hermès Bélusca-Maïto <hermes.belusca-maito(a)reactos.org>
>  Cc: ros-dev(a)reactos.org
>  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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