Re: RISC-V H-extension freeze consideration


Jonathan Behrens <behrensj@...>
 

In addition to the virtio NICs, QEMU can emulate an Intel e1000 NIC which stores packets in main memory and accesses them via DMA. I'd imagine that any other network cards of the same era would also do the same (though this is all before my time so I'm only guessing). While it is possible that QEMU also emulates some 20+ year old NICs that actually do require trapping every individual access, I'd hope nobody is using them for anything performance critical.

Jonathan


On Mon, May 31, 2021 at 3:09 PM Nick Kossifidis via lists.riscv.org <mick=ics.forth.gr@...> wrote:
Στις 2021-05-31 21:53, John Hauser έγραψε:
>> John Hauser:
>> > Maybe there's a miscommunication here.  We're talking about memory that
>> > comes with a device, which everyone considers to be part of the device,
>> > and which is mapped into the machines's address space as "I/O".  We're
>> > not talking about a device doing DMA to/from regular main memory.
>
> Greg Favor:
>> Thanks.  Then the PMA I/O attribute will apply to all accesses to the
>> device and there is no need for a "virtual I/O" bit.  (Just like in
>> x86 and
>> ARM, Svpbmt will not change this fact.  Similarly, if for example
>> VS-stage
>> says I/O type, then G-stage can't override that.)
>
> When a device is being entirely emulated by a hypervisor, where do you
> assume these "PMA I/O attributes" for the device come from, obviating
> any need for a "virtual I/O" bit in G-stage address translation?
>
>> > The scenario is a hypervisor emulating a device of the type above,
>> > where the device has its own memory in the address space that is
>> > supposed to be considered I/O.  However, for efficiency, the hypervisor
>> > doesn't want to trap every access to this emulated device memory, so it
>> > substitutes main memory for emulation purposes.
>>
>> As I've noted, this isn't possible in x86 or ARM (or what will be the
>> latest Svpbmt).
>
> I'm shocked to hear that emulating a traditional network interface card
> (NIC) on an x86 machine requires trapping every individual access to
> one of the NIC's buffers.  Have x86's really been this crippled all
> along, and I wasn't aware?
>

With virtio we have virtqueues (ring buffers) for communicating with the
host, so we don't trap on every access, here is an interesting overview:
https://www.redhat.com/en/blog/virtio-devices-and-drivers-overview-headjack-and-phone

Regards,
Nick





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