On Tue, 2024-07-16 at 15:20 +0200, Peter Hilber wrote: > On 16.07.24 14:32, David Woodhouse wrote: > > On 16 July 2024 12:54:52 BST, Peter Hilber <peter.hilber@xxxxxxxxxxxxxxx> wrote: > > > On 11.07.24 09:50, David Woodhouse wrote: > > > > On Thu, 2024-07-11 at 09:25 +0200, Peter Hilber wrote: > > > > > > > > > > IMHO this phrasing is better, since it directly refers to the state of the > > > > > structure. > > > > > > > > Thanks. I'll update it. > > > > > > > > > AFAIU if there would be abnormal delays in store buffers, causing some > > > > > driver to still see the old clock for some time, the monotonicity could be > > > > > violated: > > > > > > > > > > 1. device writes new, much slower clock to store buffer > > > > > 2. some time passes > > > > > 3. driver reads old, much faster clock > > > > > 4. device writes store buffer to cache > > > > > 5. driver reads new, much slower clock > > > > > > > > > > But I hope such delays do not occur. > > > > > > > > For the case of the hypervisor←→guest interface this should be handled > > > > by the use of memory barriers and the seqcount lock. > > > > > > > > The guest driver reads the seqcount, performs a read memory barrier, > > > > then reads the contents of the structure. Then performs *another* read > > > > memory barrier, and checks the seqcount hasn't changed: > > > > https://git.infradead.org/?p=users/dwmw2/linux.git;a=blob;f=drivers/ptp/ptp_vmclock.c;hb=vmclock#l351 > > > > > > > > The converse happens with write barriers on the hypervisor side: > > > > https://git.infradead.org/?p=users/dwmw2/qemu.git;a=blob;f=hw/acpi/vmclock.c;hb=vmclock#l68 > > > > > > My point is that, looking at the above steps 1. - 5.: > > > > > > 3. read HW counter, smp_rmb, read seqcount > > > 4. store seqcount, smp_wmb, stores, smp_wmb, store seqcount become effective > > > 5. read seqcount, smp_rmb, read HW counter > > > > > > AFAIU this would still be a theoretical problem suggesting the use of > > > stronger barriers. > > > > This seems like a bug on the guest side. The HW counter needs to be read *within* the (paired, matching) seqcount reads, not before or after. > > > > > > There would be paired reads: > > 1. device writes new, much slower clock to store buffer > 2. some time passes > 3. read seqcount, smp_rmb, ..., read HW counter, smp_rmb, read seqcount > 4. store seqcount, smp_wmb, stores, smp_wmb, store seqcount all become > effective only now > 5. read seqcount, smp_rmb, read HW counter, ..., smp_rmb, read seqcount > > I just omitted the parts which do not necessarily need to happen close to > 4. for the monotonicity to be violated. My point is that 1. could become > visible to other cores long after it happened on the local core (during > 4.). Oh, I see. That would be a bug on the device side then. And as you say, it could be fixed by using the appropriate barriers. Or my alternative of just documenting "Don't Do That Then".
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