linux-kernel - Re: [PATCH 1/4] drivers: hv: dxgkrnl: core code

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Date:   Fri, 28 Aug 2020 08:15:04 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Iouri Tarassov <iourit@...ux.microsoft.com>
Cc:     Sasha Levin <sashal@...nel.org>, kys@...rosoft.com,
        haiyangz@...rosoft.com, sthemmin@...rosoft.com, wei.liu@...nel.org,
        iourit@...rosoft.com, linux-kernel@...r.kernel.org,
        linux-hyperv@...r.kernel.org
Subject: Re: [PATCH 1/4] drivers: hv: dxgkrnl: core code

On Thu, Aug 27, 2020 at 04:45:55PM -0700, Iouri Tarassov wrote:
> 
> On 8/14/2020 5:57 AM, Greg KH wrote:
> > On Fri, Aug 14, 2020 at 08:38:53AM -0400, Sasha Levin wrote:
> > > Add support for a Hyper-V based vGPU implementation that exposes the
> > > DirectX API to Linux userspace.
> > 
> > Api questions:
> > 
> > > +struct d3dkmthandle {
> > > +	union {
> > > +		struct {
> > > +			u32 instance	:  6;
> > > +			u32 index	: 24;
> > > +			u32 unique	: 2;
> > 
> > What is the endian of this?
> > 
> > > +		};
> > > +		u32 v;
> > > +	};
> > > +};
> > > +
> > > +extern const struct d3dkmthandle zerohandle;
> > > +
> The definition is the same as on the Windows side. The driver communicates
> with a Windows host, so I do not expect any issues with endiannes. Windows
> currently runs only on the little endian platforms.

As I mentioned before, you need to document that somewhere (like maybe
preventing your code from being built on big endian systems?)

> User mode applications see this as an opaque 32 bit value (D3DKMTHANDLE). I
> prefer not to use the u32 definition to avoid mistakes when another integer
> or a 64-bit handle is assigned to the handle or  the handle is assigned to a
> 64 or 32 bit integer variable. There are many handles in the driver model
> (shared NT handle, d3dkmthandle, etc). Using a specific type allows to avoid
> assigning one handle to another.

Specific types are great, that is fine.

> > > +struct ntstatus {
> > > +	union {
> > > +		struct {
> > > +			int code	: 16;
> > > +			int facility	: 13;
> > > +			int customer	: 1;
> > > +			int severity	: 2;
> > 
> > Same here.
> > 
> > Are these things that cross the user/kernel boundry?
> > 
> > And why int on one and u32 on the other?
> > 
> > > +		};
> > > +		int v;
> > > +	};
> > > +};
> > > +
> "struct ntstatus" follows the definition for NTSTATUS on the Windows side.
> NTSTATUS is an integer where the negative values indicate errors. It is
> success otherwise. NTSTATUS is returned by the VM bus messages from host.
> IOCTLs from the driver return Linux negative error code or NTSTATUS positive
> success codes. DxCore applications expect certain positive success codes.
> DxCore is a shared library, which translates the D3DKMT* Windows interface
> to Linux ioctls. Applications link with DxCore to use a paravirtualized GPU.
> D3DKMTHANDLE is a 32-bit unsigned value (bitfield), not an integer.

Ok, again, please document this, and as these fields are crossing the
kernel/user boundry, use the correct types (hint, 'int' is never that
type).

> > > +struct winluid {
> > > +	uint a;
> > > +	uint b;
> > 
> > And now uint?  Come on, be consistent please :)
> Sorry about this. This came from the Windows size where we use UINT a lot.
> All uints will be replaced by u32 in the next patch set.

thank you.

greg k-h