> From: Jintack Lim [mailto:jint...@cs.columbia.edu] > Sent: Tuesday, June 5, 2018 8:57 PM > > Thanks, Kevin. > > On Tue, Jun 5, 2018 at 2:54 AM, Tian, Kevin <kevin.t...@intel.com> wrote: > >> From: Jintack Lim > >> Sent: Friday, June 1, 2018 11:47 AM > >> > >> Hi, > >> > >> I'm implementing Posted-interrupt functionality in vIOMMU. According > >> to Vt-d spec 5.2.3, IOMMU performs a coherent atomic read-modify- > write > >> operation of the posted-interrupt descriptor. I wonder how can we > >> achieve this considering the guest can modify the same > >> posted-interrupt descriptor anytime. Is there any existing mechanism > >> that I can use in QEMU? > >> > > > > I don't think it's possible to emulate such operation in software, unless > > you want to change guest to be cooperative. Actually it is not necessary. > > VT-d does so due to some hardware implementation consideration. > > Would you mind expanding this? I'm curious what it would be. Is it > because IOMMU can't do something like cmpxchg instructions?
I don't have further information. Above is what I was told by hardware team. > > > Since you are emulating on CPU, could just follow how CPU posted > > interrupt is conducted. If you look at SDM (29.6 Posted-Interrupt > > Processing): > > > > "There is a requirement, however, that such modifications be > > done using locked read-modify-write instructions." > > > > [instructions] means you can do update multiple times when posting an > > interrupt, as long as each update is atomic. > > Ah, that's a good point. So the unit of atomic operation doesn't need > to be the whole PI descriptor, but it can be any subset (e.g. just one > bit) of the descriptor? By looking at Linux kernel code, that seems to > be the case. > Exactly. :-) Thanks Kevin
