Messages in this thread | | | Subject | Re: [PATCH v8 2/3] tty: hvc: pass DMA capable memory to put_chars() | From | Xianting TIan <> | Date | Fri, 20 Aug 2021 16:43:33 +0800 |
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在 2021/8/20 下午2:49, Daniel Axtens 写道: > Xianting Tian <xianting.tian@linux.alibaba.com> writes: > >> As well known, hvc backend driver(eg, virtio-console) can register its >> operations to hvc framework. The operations can contain put_chars(), >> get_chars() and so on. >> >> Some hvc backend may do dma in its operations. 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): > We could also run into issues on powerpc where Andrew is working on > adding vmap-stack but the opal hvc driver assumes that it is passed a > buffer which is not in vmalloc space but in the linear mapping. So it > would be good to fix this (or more clearly document what drivers can > expect). > >> 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 future. >> >> In this patch, we make 'char out_buf[N_OUTBUF]' and 'chat out_ch' part >> of 'struct hvc_struct', so both two buf are no longer the stack memory. >> we can use it in above two cases separately. >> >> Introduce another array(cons_outbufs[]) for buffer pointers next to >> the cons_ops[] and vtermnos[] arrays. With the array, we can easily find >> the buffer, instead of traversing hp list. >> >> With the patch, we can remove the fix c4baad5029. >> >> Signed-off-by: Xianting Tian <xianting.tian@linux.alibaba.com> >> Reviewed-by: Shile Zhang <shile.zhang@linux.alibaba.com> >> struct hvc_struct { >> struct tty_port port; >> spinlock_t lock; >> int index; >> int do_wakeup; >> - char *outbuf; >> - int outbuf_size; >> int n_outbuf; >> uint32_t vtermno; >> const struct hv_ops *ops; >> @@ -48,6 +56,10 @@ struct hvc_struct { >> struct work_struct tty_resize; >> struct list_head next; >> unsigned long flags; >> + char out_ch; >> + char out_buf[N_OUTBUF] __ALIGNED__; >> + int outbuf_size; >> + char outbuf[0] __ALIGNED__; > I'm trying to understand this patch but I am finding it very difficult > to understand what the difference between `out_buf` and `outbuf` > (without the underscore) is supposed to be. `out_buf` is statically > sized and the size of `outbuf` is supposed to depend on the arguments to > hvc_alloc(), but I can't quite figure out what the roles of each one are > and their names are confusingly similiar! > > I looked briefly at the older revisions of the series but it didn't make > things much clearer. > > Could you give them clearer names?
thanks for the comments,
It is indeed not easy to understand by the name. I will change it to a proper name if we have next version patch.
Jiri Slaby is worring about the performance, because we need add two locks to protect 'out_ch' and 'out_buf' separately, the origin on-stack buffer is lockless.
I don't know whether this solution can be accepted, just waiting for Jiri's further commtents.
> > Also, looking at Documentation/process/deprecated.rst, it looks like > maybe we want to use a 'flexible array member' instead: > > .. note:: If you are using struct_size() on a structure containing a zero-length > or a one-element array as a trailing array member, please refactor such > array usage and switch to a `flexible array member > <#zero-length-and-one-element-arrays>`_ instead. > > I think we want: thanks, we should use [], not [0]. > >> + char outbuf[] __ALIGNED__; > Kind regards, > Daniel
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