On Mon, Jan 19, 2015 at 12:49 PM, Greg Kurz <gk...@linux.vnet.ibm.com> wrote: > On Mon, 19 Jan 2015 11:45:39 +0000 > Peter Maydell <peter.mayd...@linaro.org> wrote: > >> On 19 January 2015 at 11:35, Stefan Hajnoczi <stefa...@gmail.com> wrote: >> > What is the difference between the virtio_tswaps() and >> > virtio_ld/st_p() families of functions? >> >> virtio_ld/st*_p() perform a load or store from memory pointed >> to by the (host) pointer argument. >> >> virtio_tswap16 &c take a value, swap it as appropriate and >> return a value. >> >> virtio_tswap16s &c do an in-place byteswap on the value pointed >> to by the host pointer argument. >> >> (The tswap*/tswap*s pairs are a mirror of the equivalent families >> of functions bswap16/bswap16s provided by bswap.h.) >> >> The most important distinction here is that the ld/st functions >> will handle possibly unaligned accesses, whereas the tswap >> functions will not. The former are intended for "fish this value >> out from some guest RAM" and the latter for "we have a structure >> we've copied from guest RAM already via memcpy and want to swizzle >> the entries in it". >> >> -- PMM >> > > Heh, not sure I would have provided such a comprehensive answer. :) > > Stefan, > > Feeling curious, what was the intent behind your question ?
I'm converting hw/virtio/dataplane/vring.c to use virtio_ld/st() accessors so that KVM on Power supports bi-endian mode. I noticed that devices use both virtio_tswap() and virtio_ld/st() in different places. My main worry was that they are implemented slightly different (how they combine #ifdefs and virtio_access_is_big_endian()) and that I was missing something. But it seems in my case there is no difference between the two. Stefan
