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[Qemu-devel] Re: [PATCH 2 of 5] add can_dma/post_dma for direct IO

From: Avi Kivity
Subject: [Qemu-devel] Re: [PATCH 2 of 5] add can_dma/post_dma for direct IO
Date: Sun, 14 Dec 2008 21:59:29 +0200
User-agent: Thunderbird (X11/20081119)

Andrea Arcangeli wrote:
On Sun, Dec 14, 2008 at 07:01:38PM +0200, Avi Kivity wrote:
Actually, with Xen, RAM may be unmapped due do Xen limitations when qemu runs on dom0 mode. But I think map/unmap makes sense even disregarding

I realize xen 32bit has issues... Qemu/KVM 32bit also has the same
issues but there's no point in 2009 (that's when this stuff could go
productive) in trying to run guests with >2G of ram on a 32bit
host. The issues emerges (I guess with xen too) in trying to run those
obsolete hardware configurations. Even the atom and extremely low
power athlon have 64bit capability, and on embedded that runs a real
32bit cpu I can't see how somebody would want to run a >2G guest.

kvm and Xen actually have different issues for 32-bit. For kvm, supporting >2G on 32-bits is possible but messy and pointless, so we chose not to do it. For Xen, this is a critical performance issue as 64-bit userspace in pv guests is quite slow. So dom0 runs as a 32-bit guest

Newer Xen shouldn't have this problem though; it runs qemu in kernel mode in a dedicated 64-bit domain.

Xen: if we add memory hotunplug, we need to make sure we don't unplug memory that has pending dma operations on it. map/unmap gives us the opportunity to refcount memory slots.

So memory hotunplug here is considered differently than the real
memory hotplug emulation that simulates removing dimm on the
hardware. This is just the xen trick to handle >4G guest on a 32bit
address space? Well that's just the thing I'm not interested to
support. When 64bit wasn't mainstream it made some sense, these days
it's good enough if we can boot any guest OS (including 64bit ones) on
a 32bit build, but trying to run guests OS with >2G of ram doesn't
look useful.

Leaving Xen aside, memory hotunplug requires that we tell the memory when it's used and when it isn't.

We can't get all dma to stop during hotunplug, since net rx operations are long-running (infinite if there is no activity on the link).

IMO, we do want map/unmap, but this would be just a rename of can_dma and friends, and wouldn't have at this time any additional functionality. Bouncing has to happen where we have the ability to schedule the actual operation, and that's clearly not map/unmap.

It would be a bit more than a rename, also keep in mind that in the
longer term as said we need to build the iovec in the exec.c path,
it's not enough to return a void *, I like to support a not 1:1 flat
space to avoid wasting host virtual address space with guest memory
holes. But that's about it, guest memory has to be always mapped, just
not with a 1:1 mapping, and surely not with a per-page array that
translates each page physical address to a host virtual address, but
with ranges. So this map thing that returns a 'void *' won't be there
for long even if I rename.

If it returns an iovec, still that doesn't change how it works. I like the symmetry of map()/unmap() and the lock/unlock semantics (like kmap_atomic/kunmap_atomic and a myriad other get/put pairs).

[There's actually a language that supports this idiom, but that's a different flamewar]

I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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