Re: [PATCH] usb: allow max 8192 bytes for desc

[Philippe Mathieu-Daudé]

On 1/11/22 13:27, zhenwei pi wrote:
> On 1/11/22 8:25 PM, Daniel P. Berrangé wrote:
>> On Tue, Jan 11, 2022 at 12:21:42PM +0000, Peter Maydell wrote:
>>> On Tue, 11 Jan 2022 at 10:54, zhenwei pi <pizhenwei@bytedance.com>
>>> wrote:
>>>>
>>>> A device of USB video class usually uses larger desc structure, so
>>>> use larger buffer to avoid failure. (dev-video.c is ready)
>>>>
>>>> Allocating memory dynamically by g_malloc of the orignal version of
>>>> this change, Philippe suggested just using the stack. Test the two
>>>> versions of qemu binary, the size of stack gets no change.
>>>>
>>>> CC: Philippe Mathieu-Daudé <f4bug@amsat.org>
>>>> Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>
>>>> ---
>>>>   hw/usb/desc.c | 2 +-
>>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/hw/usb/desc.c b/hw/usb/desc.c
>>>> index 8b6eaea407..57d2aedba1 100644
>>>> --- a/hw/usb/desc.c
>>>> +++ b/hw/usb/desc.c
>>>> @@ -632,7 +632,7 @@ int usb_desc_get_descriptor(USBDevice *dev,
>>>> USBPacket *p,
>>>>       bool msos = (dev->flags & (1 << USB_DEV_FLAG_MSOS_DESC_IN_USE));
>>>>       const USBDesc *desc = usb_device_get_usb_desc(dev);
>>>>       const USBDescDevice *other_dev;
>>>> -    uint8_t buf[256];
>>>> +    uint8_t buf[8192];
>>>>       uint8_t type = value >> 8;
>>>>       uint8_t index = value & 0xff;
>>>>       int flags, ret = -1;
>>>
>>> I think 8K is too large to be allocating as an array on
>>> the stack, so if we need this buffer to be larger we should
>>> switch to some other allocation strategy for it.
>>
>> IIUC, querying USB device descriptors is not a hot path, so using
>> heap allocation feels sufficient.
>>
> Yes, I tested this a lot, and found that it's an unlikely code path:
> 1, during guest startup, guest tries to probe device.
> 2, run 'lsusb' command in guest(or other similar commands).

Sorry, I thought this was a hot path without looking at the code.

> The original patch and context link:
> https://patchwork.kernel.org/project/qemu-devel/patch/20211227142734.691900-5-pizhenwei@bytedance.com/
> 
>>
>> Regards,
>> Daniel
>>
>