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[Qemu-devel] Re: [PATCH 2/3] Assume PPC64 host on PPC32 KVM


From: Alexander Graf
Subject: [Qemu-devel] Re: [PATCH 2/3] Assume PPC64 host on PPC32 KVM
Date: Fri, 24 Jul 2009 13:23:30 +0200


On 24.07.2009, at 13:17, Jan Kiszka wrote:

Alexander Graf wrote:

On 24.07.2009, at 12:59, Jan Kiszka wrote:

Alexander Graf wrote:
When talking to the kernel about dirty maps, we need to find out which
bits were actually set. This is done by set_bit and test_bit like
functiontality which uses the "long" variable type.

Now, with PPC32 userspace and PPC64 kernel space (which is pretty
common),
we can't interpret the bits properly anymore, because we think long is
32 bits wide.

So for PPC dirty bitmap analysis, let's just assume we're always running on a PPC64 host. Currently there is no dirty bitmap implementation for
PPC32 / PPCEMB anyways.

Unbreaks dirty logging on PPC.

Signed-off-by: Alexander Graf <address@hidden>
---
kvm-all.c |    6 ++++++
1 files changed, 6 insertions(+), 0 deletions(-)

diff --git a/kvm-all.c b/kvm-all.c
index 824bb4c..bfaa623 100644
--- a/kvm-all.c
+++ b/kvm-all.c
@@ -357,7 +357,13 @@ int
kvm_physical_sync_dirty_bitmap(target_phys_addr_t start_addr,
       for (phys_addr = mem->start_addr, addr = mem->phys_offset;
            phys_addr < mem->start_addr + mem->memory_size;
phys_addr += TARGET_PAGE_SIZE, addr += TARGET_PAGE_SIZE) {
+#ifdef HOST_PPC
+ /* Big endian keeps us from having different long sizes
in user and
+              * kernel space, so assume we're always on ppc64. */
+            uint64_t *bitmap = (uint64_t *)d.dirty_bitmap;
+#else
           unsigned long *bitmap = (unsigned long *)d.dirty_bitmap;
+#endif
           unsigned nr = (phys_addr - mem->start_addr) >>
TARGET_PAGE_BITS;
           unsigned word = nr / (sizeof(*bitmap) * 8);
           unsigned bit = nr % (sizeof(*bitmap) * 8);

This rather screams for a generic fix. Current code assumes
sizeof(unsigned long) == 8. That should already break on 32-bit x86
hosts. So either do (sizeof(*bitmap) * sizeof(unsigned long)) or switch
to uint64_t - but for ALL hosts.

I don't see where that would break. The kernel treats the array as
ulong*, userspace treats it as ulong* and set_bit in kernel does
bitmap[word] |= (1 << bit). So as long as userspace long and kernel long
are the same, it works.

In fact - it should even work out with little endian and different ulong
sizes. It just breaks on BE.

Err, yes, forget it.

But let's help me understanding the actual problem: Do you have
different ulong sizes in your scenario? Why? Is it a compat issue of
32-bit userland on 64-bit kernel?

32-bit userland on 64-bit kernel.

kernel: sizeof(ulong) = 8
userspace: sizeof(ulong) = 4

now, with big endian, a "1" is on the rightmost byte - which means looking at the bytes it's

kernel: byte[7]
userspace: byte[3]

So if you set bit nr "1" with the current logic, the kernel would set bit "1" (in the first 8 bytes), userspace would read bit "1" in the second byte, thus 32 + 1.

On little endian, the lower word is on the first 4 bytes, so it would still be bit "1" in the first byte.

Alex





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