Re: [PATCH v6 1/2] tty: hvc: pass DMA capable memory to put_chars()

[Xianting TIan]

在 2021/8/13 下午3:27, Greg KH 写道:
> On Thu, Aug 12, 2021 at 05:45:31PM +0800, Xianting Tian wrote:
> > As well known, hvc backend can register its opertions to hvc backend.
> > the opertions contain put_chars(), get_chars() and so on.
> >
> > Some hvc backend may do dma in its opertions. eg, put_chars() of
> > virtio-console. But in the code of hvc framework, it may pass DMA
> > incapable memory to put_chars() under a specific configuration, which
> > is explained in commit c4baad5029(virtio-console: avoid DMA from stack):
> > 1, c[] is on stack,
> >     hvc_console_print():
> > 	char c[N_OUTBUF] __ALIGNED__;
> > 	cons_ops[index]->put_chars(vtermnos[index], c, i);
> > 2, ch is on stack,
> >     static void hvc_poll_put_char(,,char ch)
> >     {
> > 	struct tty_struct *tty = driver->ttys[0];
> > 	struct hvc_struct *hp = tty->driver_data;
> > 	int n;
> >
> > 	do {
> > 		n = hp->ops->put_chars(hp->vtermno, &ch, 1);
> > 	} while (n <= 0);
> >     }
> >
> > Commit c4baad5029 is just the fix to avoid DMA from stack memory, which
> > is passed to virtio-console by hvc framework in above code. But I think
> > the fix is aggressive, it directly uses kmemdup() to alloc new buffer
> > from kmalloc area and do memcpy no matter the memory is in kmalloc area
> > or not. But most importantly, it should better be fixed in the hvc
> > framework, by changing it to never pass stack memory to the put_chars()
> > function in the first place. Otherwise, we still face the same issue if
> > a new hvc backend using dma added in the furture.
> >
> > We make 'char c[N_OUTBUF]' part of 'struct hvc_struct', so hp->c is no
> > longer the stack memory. we can use it in above two cases.
> >
> > Other fix is use L1_CACHE_BYTES as the alignment, use 'sizeof(long)' as
> > dma alignment is wrong. And use struct_size() to calculate size of
> > hvc_struct.
> >
> > Introduce another array(cons_outbuf[]) for the hp->c pointers next to
> > the cons_ops[] and vtermnos[] arrays.
> >
> > With the patch, we can remove the fix c4baad5029.
> >
> > Signed-off-by: Xianting Tian <xianting.tian@xxxxxxxxxxxxxxxxx>
> > Tested-by: Xianting Tian <xianting.tian@xxxxxxxxxxxxxxxxx>
> As the build shows, you obviously did not test this code :(
>
> Also, no need to add a tested-by line as that should be implicit if you
> wrote and signed off on it.
>
> I am going to ask you to get some help from some other developers at
> your company, and get them to test and sign off on this series before
> sending it out again, as there seems to be a bit of a disconnect as to
> what is actually needed to do when sending a patch for us to review.
>
> That is now a requirement for us to be able to take your changes here.
>
> thanks,

Sorry for this.

I tested V1-V4,  But for V6, I take it for granted that there is no 
problem when I just switch to use array(cons_outbuf[]).  I indeed didn't 
test it:(

I will test it and find virtualization test expert to test again before 
sending next patch.

> greg k-h