Re: [win-pv-devel] Next signed release - support for readonly disks?

[Chris McClymont <chris@xxxxxxxxxxxx>]

> In my experience an NTFS filesystem requires a disk to be read/write 
> otherwise it will not function... certainly this is true of a system disk. 
> Are you using a different filesystem? TBH I think it's quite reasonable for 
> Windows to assume that a non-removable medium calling itself is a disk is 
> read/write.

Readonly disks appear to behave as expected for me using Windows
Server 2012 with the GPLPV drivers signed by EJB Digital, tested using
both VFAT and NTFS. In the Disk Management UI, the disk appears as
Read Only where it normally says Online. For some reason, the option
to add a drive letter is not there, but I can mount them using
diskpart. Files can be read but trying to paste a file into the
mounted readonly disk results in an error dialog:
"The disk is write-protected. ... Remove the write-protection or use
another disk."  Try Again / Skip / Cancel

I believe this is achieved by setting the Write Protected flag in the
SCSI parameters in response to the Sense command.
It looks like it is set here in the GPLPV code:
http://xenbits.xensource.com/ext/win-pvdrivers/file/2d392ecdd366/xenvbd_common/common_miniport.h#l497

These disks have "EJECTSUPPORTED" and also "SURPRISEREMOVALOK" as
capabilities (In both GPLPV and this project) so I think they are
classified as removable.

> This fix was unintended - and it does seem possible to boot with read-only 
> non-boot disks attached. It's not a fully tested setup, and the frontend 
> offers no protection against writes to read only disks, so the results are 
> undefined
Yeah, my testing shows that writes appear to work, but after
refreshing the disk or folder the written files disappear.

Thanks Paul and Owen, I will either keep using the GPLPV drivers or
look into feasibility of patching in the SCSI level write protection.

Regards,
Chris McClymont


On Tue, Oct 17, 2017 at 1:46 AM, Paul Durrant <Paul.Durrant@xxxxxxxxxx> wrote:
>> -----Original Message-----
>> From: win-pv-devel [mailto:win-pv-devel-bounces@xxxxxxxxxxxxxxxxxxxx] On
>> Behalf Of Chris McClymont
>> Sent: 14 October 2017 02:19
>> To: win-pv-devel@xxxxxxxxxxxxxxxxxxxx
>> Subject: [win-pv-devel] Next signed release - support for readonly disks?
>>
>> Hi everyone,
>>
>> Support for readonly disks was fixed in xenvbd commit
>> 3bb41b49ff66433ab5ebb5d36d24946f40390d7c. When is this likely to make
>> it into a signed release?
>>
>>
>> I'm working on Windows templates for Xen and would love to use the
>> newer Xen Project drivers signed by the Linux Foundation, rather than
>> the EJB Digital or others floating around. But I have a requirement to
>> support readonly disks, which don't work in the 8.2.0 release.
>> Readonly disks appear in device manage only when emulated, and
>> disappear (even with Show Hidden Devices) after the PV drivers are
>> installed.
>>
>> The commit I mentioned is somewhere between 8.2.0 and master.
>> Previously, pdo.c would set ScsiStatus to SCSI_ABORT if
>> (DiskInfo->DiskInfo & VDISK_READONLY). Now, pdo.c is called target.c
>> and the commit removes the readonly limitation.
>>
>
> Hi Chris,
>
> Owen may have more to say, but I assume this was an unintended consequence of 
> the patch.
>
> In my experience an NTFS filesystem requires a disk to be read/write 
> otherwise it will not function... certainly this is true of a system disk. 
> Are you using a different filesystem? TBH I think it's quite reasonable for 
> Windows to assume that a non-removable medium calling itself is a disk is 
> read/write.
>
>> I'm wondering a few things:
>>   1) When is this commit likely to make it into a signed release
>
> It's not currently a back-port candidate and there is no firm timeline for 
> releasing 9.0 drivers.
>
>>   2) Is there a roadmap? I.e. How is it decided when to make releases
>> and which features/fixes to include
>
> Because the Xen Project does not have dedicated infrastructure for testing 
> the PV drivers, we generally rely on the testing done by Citrix XenServer and 
> therefore releases tend to be aligned with updates made to the Citrix branded 
> variant of the drivers.
>
>>   3) How can I search this mailing list? It doesn't seem to be
>> included on http://xen.markmail.org/search/
>>
>
> You can see the mailing list archives at 
> https://lists.xenproject.org/archives/html/win-pv-devel. As for searching via 
> markmail... I have requested that the list be added in the past, but I'll 
> have another go.
>
> Cheers,
>
>   Paul
>
>> Thanks,
>> Chris McClymont
>>
>> _______________________________________________
>> win-pv-devel mailing list
>> win-pv-devel@xxxxxxxxxxxxxxxxxxxx
>> https://lists.xenproject.org/cgi-bin/mailman/listinfo/win-pv-devel

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