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[Qemu-devel] Re: [PATCH 6/7] Set slots more carefully


From: Alexander Graf
Subject: [Qemu-devel] Re: [PATCH 6/7] Set slots more carefully
Date: Fri, 17 Jul 2009 16:23:05 +0200


On 17.07.2009, at 16:18, Jan Kiszka wrote:

Alexander Graf wrote:

On 17.07.2009, at 15:48, Jan Kiszka wrote:

Alexander Graf wrote:
KVM only supports slot sizes of PAGE_SIZE granilarity. On PPC the OS sets the framebuffer to some odd size though, causing the current code
to simply abort().

So let's bet graceful here. We can just allocate memory sizes that
are of
PAGE_SIZE granularity and everything that exceeds that comes in as
MMIO and
gets handled too - just slower.

Signed-off-by: Alexander Graf <address@hidden>
---
kvm-all.c |    2 +-
1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kvm-all.c b/kvm-all.c
index 961fa32..60b76cf 100644
--- a/kvm-all.c
+++ b/kvm-all.c
@@ -134,7 +134,7 @@ static int kvm_set_user_memory_region(KVMState
*s, KVMSlot *slot)

   mem.slot = slot->slot;
   mem.guest_phys_addr = slot->start_addr;
-    mem.memory_size = slot->memory_size;
+    mem.memory_size = slot->memory_size & ~(TARGET_PAGE_SIZE - 1);

TARGET_PAGE_MASK? And I bet you want to round up here...


Eh - yeah. Same thing, no?

Just look at its definition...


And no, I don't want to round up. After the memory backed slot could
easily be real MMIO space. I had such strange configurations with the
ESCC overlapping in the same page as graphic memory once.

Overlapping is handled by the kvm layer in user space. For sure, if
there is a mapping conflict, we are in trouble (kvm-wise) as the second
request overwrites the mapping type of the overlapping page(s).


Better be safe than sorry.

No, something wrong. If you cut off the odd "overhead", you effectively
exclude that page. So either the caller should not include it in the
first place as it is unused or kvm should try to map the whole page just like the rest of the slot. If there remains an unresolvable conflict, we
need to enhance the slot management with some sub-page dispatching
mechanism.

Not mapped = MMIO

MMIO then gets passed to userspace which puts it into qemu's read/ write functions which then decide if it's real MMIO or just a plain RAM access.

So all we get is a couple more MMIO accesses, right?

Alex




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