Re: [Qemu-devel] vIOMMU Posted-interrupt implementation - atomic operation?
On Wed, Jun 6, 2018 at 2:56 AM, Tian, Kevin wrote: >> 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 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. Ah, I see. Thanks! > >> >> > 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. :-) Cool. Thanks for the confirmation. Thanks, Jintack > > Thanks > Kevin
Re: [Qemu-devel] vIOMMU Posted-interrupt implementation - atomic operation?
> 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 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
Re: [Qemu-devel] vIOMMU Posted-interrupt implementation - atomic operation?
Thanks, Kevin. On Tue, Jun 5, 2018 at 2:54 AM, Tian, Kevin 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? > 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. Best, Jintack > > Thanks > Kevin >
Re: [Qemu-devel] vIOMMU Posted-interrupt implementation - atomic operation?
> 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. 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. Thanks Kevin
[Qemu-devel] vIOMMU Posted-interrupt implementation - atomic operation?
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? Thanks, Jintack
