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

["Daniel P. Berrangé" <berrange@redhat.com>]

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.


Regards,
Daniel
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