Re: [RFC 1/2] vhost: IFC VF hardware operation layer

[Jason Wang]

On 2019/10/22 äå12:31, Simon Horman wrote:
> On Mon, Oct 21, 2019 at 05:55:33PM +0800, Zhu, Lingshan wrote:
> > On 10/16/2019 5:53 PM, Simon Horman wrote:
> > > Hi Zhu,
> > >
> > > thanks for your patch.
> > >
> > > On Wed, Oct 16, 2019 at 09:10:40AM +0800, Zhu Lingshan wrote:
> ...
>
> > > > +static void ifcvf_read_dev_config(struct ifcvf_hw *hw, u64 offset,
> > > > +		       void *dst, int length)
> > > > +{
> > > > +	int i;
> > > > +	u8 *p;
> > > > +	u8 old_gen, new_gen;
> > > > +
> > > > +	do {
> > > > +		old_gen = ioread8(&hw->common_cfg->config_generation);
> > > > +
> > > > +		p = dst;
> > > > +		for (i = 0; i < length; i++)
> > > > +			*p++ = ioread8((u8 *)hw->dev_cfg + offset + i);
> > > > +
> > > > +		new_gen = ioread8(&hw->common_cfg->config_generation);
> > > > +	} while (old_gen != new_gen);
> > > Would it be wise to limit the number of iterations of the loop above?
> > Thanks but I don't quite get it. This is used to make sure the function
> > would get the latest config.
> I am worried about the possibility that it will loop forever.
> Could that happen?
>
> ...


My understanding is that the function here is similar to virtio config 
generation [1]. So this can only happen for a buggy hardware.

Thanks

[1] 
https://docs.oasis-open.org/virtio/virtio/v1.1/csprd01/virtio-v1.1-csprd01.html 
Section 2.4.1


> > > > +static void io_write64_twopart(u64 val, u32 *lo, u32 *hi)
> > > > +{
> > > > +	iowrite32(val & ((1ULL << 32) - 1), lo);
> > > > +	iowrite32(val >> 32, hi);
> > > > +}
> > > I see this macro is also in virtio_pci_modern.c
> > >
> > > Assuming lo and hi aren't guaranteed to be sequential
> > > and thus iowrite64_hi_lo() cannot be used perhaps
> > > it would be good to add a common helper somewhere.
> > Thanks, I will try after this IFC patchwork, I will cc you.
> Thanks.
>
> ...