Linux Virtualization - Apr 2023 - [PATCH resend 1/2] vdpa/snet: support getting and setting VQ state

On Mon, Apr 3, 2023 at 5:18?PM Alvaro Karsz <alvaro.karsz at
solid-run.com> wrote:
>
> Hi Jason,
>
> > > +       /* Overwrite the control register with the new buffer
size (in 4B words) */
> > > +       snet_write_ctrl(regs, buf_words);
> > > +       /* Use a memory barrier, this must be written before the
opcode register. */
> > > +       wmb();
> >
> >
> > At least you need to use smp_wmb() but if you want to serialize MMIO
> > writes you can simply use spinlocks after the removal of mmiowb
> > work[1].
> >
>
> I'm not sure how a spinlock can help in this case.
> The entire control mechanism is protected by the spinlock
snet->ctrl_lock, so multiple CPUs won't use it simultaneously.
>
> > Note that Documentation/memory-barriers.txt said:
> >
> >         1. All readX() and writeX() accesses to the same peripheral
are ordered
> >            with respect to each other. This ensures that MMIO register
accesses
> >            by the same CPU thread to a particular device will arrive
in program
> >            order.
> >
>
> So it will arrive to the pci subsystem in program order, but the pci
subsystem may reorder, right?
This is not what I read from the above doc. It said "to a particular
device will arrive in program order".
> I can just issue a read after the writes, something like:
>
> write_control
> read_control
> write op
> read op
>
> What do you think?
This should work.

Thanks
>
> > > +       /* Clear the control register - clear the error code if
previous control operation failed */
> > > +       snet_write_ctrl(regs, 0);
> > > +
> > > +       /* Write opcode and VQ idx */
> > > +       val = opcode | (vq_idx << 16);
> > > +       snet_write_op(regs, val);
> >
> > I guess we need to serialize two writes here as well.
>
> Same here.
>
> Thanks
>
>

> > So it will arrive to the pci subsystem in program order, but the pci
subsystem may reorder, right?
> 
> This is not what I read from the above doc. It said "to a particular
> device will arrive in program order".
In this case, I can just remove the wmb and that's it, right?

Thanks