Re: [PATCH 2/7] guest_memfd: Introduce an object to manage the guest-memfd with RamDiscardManager

[Peter Xu <peterx@redhat.com>]

On Wed, Jan 22, 2025 at 05:41:31PM +0800, Xu Yilun wrote:
> On Wed, Jan 22, 2025 at 03:30:05PM +1100, Alexey Kardashevskiy wrote:
> > 
> > 
> > On 22/1/25 02:18, Peter Xu wrote:
> > > On Tue, Jun 25, 2024 at 12:31:13AM +0800, Xu Yilun wrote:
> > > > On Mon, Jan 20, 2025 at 03:46:15PM -0500, Peter Xu wrote:
> > > > > On Mon, Jan 20, 2025 at 09:22:50PM +1100, Alexey Kardashevskiy wrote:
> > > > > > > It is still uncertain how to implement the private MMIO. Our assumption
> > > > > > > is the private MMIO would also create a memory region with
> > > > > > > guest_memfd-like backend. Its mr->ram is true and should be managed by
> > > > > > > RamdDiscardManager which can skip doing DMA_MAP in VFIO's region_add
> > > > > > > listener.
> > > > > > 
> > > > > > My current working approach is to leave it as is in QEMU and VFIO.
> > > > > 
> > > > > Agreed.  Setting ram=true to even private MMIO sounds hackish, at least
> > > > 
> > > > The private MMIO refers to assigned MMIO, not emulated MMIO. IIUC,
> > > > normal assigned MMIO is always set ram=true,
> > > > 
> > > > void memory_region_init_ram_device_ptr(MemoryRegion *mr,
> > > >                                         Object *owner,
> > > >                                         const char *name,
> > > >                                         uint64_t size,
> > > >                                         void *ptr)

[1]

> > > > {
> > > >      memory_region_init(mr, owner, name, size);
> > > >      mr->ram = true;
> > > > 
> > > > 
> > > > So I don't think ram=true is a problem here.
> > > 
> > > I see.  If there's always a host pointer then it looks valid.  So it means
> > > the device private MMIOs are always mappable since the start?
> > 
> > Yes. VFIO owns the mapping and does not treat shared/private MMIO any
> > different at the moment. Thanks,
> 
> mm.. I'm actually expecting private MMIO not have a host pointer, just
> as private memory do.
> 
> But I'm not sure why having host pointer correlates mr->ram == true.

If there is no host pointer, what would you pass into "ptr" as referenced
at [1] above when creating the private MMIO memory region?

OTOH, IIUC guest private memory finally can also have a host pointer (aka,
mmap()-able), it's just that even if it exists, accessing it may crash QEMU
if it's private.

Thanks,

-- 
Peter Xu