Re: [PATCH] net/hns3: support disable IOVA as PA mode

[Bruce Richardson <bruce.richardson@intel.com>]

On Mon, Feb 20, 2023 at 04:30:20PM +0100, Thomas Monjalon wrote:
> 20/02/2023 16:07, Morten Brørup:
> > With the new viewing angle, the current define RTE_IOVA_AS_PA makes more sense to me now than before. So we should probably stick with it, rather than introduce something that might confuse developers who already have the same viewing angle.
> > 
> > But it still seems counterintuitive to me that disabling some feature ("enable_iova_as_pa") is not supported throughout DPDK; the logic seems inverted. Apparently, it also makes it difficult to assign good titles to patches that support disabling such a feature. :-)
> > 
> > <irony>
> > On the positive side, since everything supports this "enable_iova_as_pa" feature, we don't need to add it to the PMD feature list. If the logic wasn't inverted like this, the PMD feature list should probably reflect which PMDs supported the "iova_as_va_only" compile time option. ;-)
> > </irony>
> 
> That's a change I would like to do:
> The Meson variable in the drivers should be "support_iova_as_va"
> and would mean we can compile the driver when "enable_iova_as_pa" is false.
> 
All drivers (that I am aware of) support iova_as_va. What is missing is
drivers supporting "iova_as_va_only". Any reference to va without the word
"only" on it will be misleading.

A third way of looking at it, is to work with the fact that the reason
drivers require changes to support this "va_only" mode, ro no-pa mode, is
due to the fact that the mbuf no longer tracks iovas and only VAs.
Therefore, we can have a variable called "require_iova_in_mbuf", which
would hopefully cut through this whole va vs pa addition/subtraction mess.
What do you think?

/Bruce