[Qemu-devel] Re: [PATCH 4/7] ide: use the PCI memory access interface

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Sep 02, 2010 at 06:01:35PM +0300, Eduard - Gabriel Munteanu wrote:
> On Thu, Sep 02, 2010 at 12:58:13PM +0300, Michael S. Tsirkin wrote:
> > On Thu, Sep 02, 2010 at 12:12:00PM +0300, Eduard - Gabriel Munteanu wrote:
> > > On Thu, Sep 02, 2010 at 08:19:11AM +0300, Michael S. Tsirkin wrote:
> > > > On Sat, Aug 28, 2010 at 05:54:55PM +0300, Eduard - Gabriel Munteanu wrote:
> > 
> > I don't insist on this solution, but what other way do you propose to
> > avoid the overhead for everyone not using an iommu?
> > I'm all for a solution that would help iommu as well,
> > but one wasn't yet proposed.
> > 
> 
> Hm, we could get even better performance by simply making the IOMMU a
> compile-time option. It also avoids problems in case some device hasn't
> been converted yet, and involves little to no tradeoffs. What do you
> think?
> 
> AFAICT, there are few uses for the IOMMU besides development and
> avantgarde stuff, as you note. So distributions can continue supplying
> prebuilt QEMU/KVM packages compiled with the IOMMU turned off for the
> time being. The only practical (commercial) use right now would be in
> the case of private virtual servers, which could be divided further into
> nested guests (though real IOMMU hardware isn't widespread yet).
> 
> Blue Swirl, in the light of this, do you agree on making it a
> compile-time option?
> 
> > > > >  static inline IDEState *idebus_active_if(IDEBus *bus)
> > > > >  {
> > > > >      return bus->ifs + bus->unit;
> > > > > diff --git a/hw/ide/macio.c b/hw/ide/macio.c
> > > > > index bd1c73e..962ae13 100644
> > > > > --- a/hw/ide/macio.c
> > > > > +++ b/hw/ide/macio.c
> > > > > @@ -79,7 +79,7 @@ static void pmac_ide_atapi_transfer_cb(void *opaque, int ret)
> > > > >  
> > > > >      s->io_buffer_size = io->len;
> > > > >  
> > > > > -    qemu_sglist_init(&s->sg, io->len / MACIO_PAGE_SIZE + 1);
> > > > > +    qemu_sglist_init(&s->sg, io->len / MACIO_PAGE_SIZE + 1, NULL, NULL, NULL);
> > > > >      qemu_sglist_add(&s->sg, io->addr, io->len);
> > > > >      io->addr += io->len;
> > > > >      io->len = 0;
> > > > > @@ -141,7 +141,7 @@ static void pmac_ide_transfer_cb(void *opaque, int ret)
> > > > >      s->io_buffer_index = 0;
> > > > >      s->io_buffer_size = io->len;
> > > > >  
> > > > > -    qemu_sglist_init(&s->sg, io->len / MACIO_PAGE_SIZE + 1);
> > > > > +    qemu_sglist_init(&s->sg, io->len / MACIO_PAGE_SIZE + 1, NULL, NULL, NULL);
> > > > >      qemu_sglist_add(&s->sg, io->addr, io->len);
> > > > >      io->addr += io->len;
> > > > >      io->len = 0;
> > > > > diff --git a/hw/ide/pci.c b/hw/ide/pci.c
> > > > > index 4d95cc5..5879044 100644
> > > > > --- a/hw/ide/pci.c
> > > > > +++ b/hw/ide/pci.c
> > > > > @@ -183,4 +183,11 @@ void pci_ide_create_devs(PCIDevice *dev, DriveInfo **hd_table)
> > > > >              continue;
> > > > >          ide_create_drive(d->bus+bus[i], unit[i], hd_table[i]);
> > > > >      }
> > > > > +
> > > > > +    for (i = 0; i < 2; i++) {
> > > > > +        d->bmdma[i].rw = (void *) pci_memory_rw;
> > > > > +        d->bmdma[i].map = (void *) pci_memory_map;
> > > > > +        d->bmdma[i].unmap = (void *) pci_memory_unmap;
> > > > > +        d->bmdma[i].opaque = dev;
> > > > > +    }
> > > > >  }
> > > > 
> > > > These casts show something is wrong with the API, IMO.
> > > > 
> > > 
> > > Hm, here's an oversight on my part: I think I should provide explicit
> > > bmdma hooks, since pcibus_t is a uint64_t and target_phys_addr_t is a
> > > uint{32,64}_t depending on the guest machine, so it might be buggy on
> > > 32-bit wrt calling conventions. But that introduces yet another
> > > non-inlined function call :-(. That would drop the (void *) cast,
> > > though.
> > > 
> > > 
> > > 	Eduard
> > 
> > So we get away with it without casts but only because C compiler
> > will let us silently convert the types, possibly discarding
> > data in the process. Or we'll add a check that will try and detect
> > this, but there's no good way to report a DMA error to user.
> > IOW, if our code only works because target fits in pcibus, what good
> > is the abstraction and using distinct types?
> > 
> > This is why I think we need a generic DMA APIs using dma addresses.
> 
> The API was made so that it doesn't report errors. That's because the
> PCI bus doesn't provide any possibility of doing so (real devices can't
> retry transfers in case an I/O page fault occurs).

This is what I am saying. We can't deal with errors.

> In my previous generic IOMMU layer implementation pci_memory_*()
> returned non-zero on failure, but I decided to drop it when switching to
> a PCI-only (rather a PCI-specific) approach.
> 
> In case target_phys_addr_t no longer fits in pcibus_t by a simple
> implicit conversion, those explicit bmdma hooks I was going to add will
> do the necessary conversions.
> 
> The idea of using distinct types is two-fold: let the programmer know
> not to rely on them being the same thing, and let the compiler prevent
> him from shooting himself in the foot (like I did). Even if there is a
> dma_addr_t, some piece of code still needs to provide glue and
> conversion between the DMA code and bus-specific code.
> 
> 
> 	Eduard

Nothing I see here is bus-specific, really. Without an mmu addresses
that make sense are target addresses, with iommu - whatever iommu
supports. So make iommu work with dma_addr_t and do the conversion.

-- 
MST