On Fri, Jul 08, 2016 at 04:01:36PM +0100, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <dgilb...@redhat.com>
>
> Currently QEMU sets the x86 number of physical address bits to the
> magic number 40. This is only correct on some small AMD systems;
> Intel systems tend to have 36, 39, 46 bits, and large AMD systems
> tend to have 48.
>
> Having the value different from your actual hardware is detectable
> by the guest and in principal can cause problems;
> The current limit of 40 stops TB VMs being created by those lucky
> enough to have that much.
>
> This patch lets you set the physical bits by a cpu property but
> defaults to the same 40bits which matches TCGs setup.
>
> I've removed the ancient warning about the 42 bit limit in exec.c;
> I can't find that limit in there and no one else seems to know where
> it is.
I think I've found it, buried in ancient commits:
commit 5270589032f450ae7c3448730855aa18ff68ccff
Author: Richard Henderson <r...@twiddle.net>
Date: Wed Mar 10 14:33:23 2010 -0800
Move TARGET_PHYS_ADDR_SPACE_BITS to target-*/cpu.h.
Removes a set of ifdefs from exec.c.
Introduce TARGET_VIRT_ADDR_SPACE_BITS for all targets other
than Alpha. This will be used for page_find_alloc, which is
supposed to be using virtual addresses in the first place.
Signed-off-by: Richard Henderson <r...@twiddle.net>
[...]
--- a/exec.c
+++ b/exec.c
[...]
-#elif defined(TARGET_X86_64)
-#define TARGET_PHYS_ADDR_SPACE_BITS 42
-#elif defined(TARGET_I386)
-#define TARGET_PHYS_ADDR_SPACE_BITS 36
[...]
--- a/target-i386/cpu.h
+++ b/target-i386/cpu.h
[...]
+#ifdef TARGET_X86_64
+#define TARGET_PHYS_ADDR_SPACE_BITS 52
+/* ??? This is really 48 bits, sign-extended, but the only thing
+ accessible to userland with bit 48 set is the VSYSCALL, and that
+ is handled via other mechanisms. */
+#define TARGET_VIRT_ADDR_SPACE_BITS 47
+#else
+#define TARGET_PHYS_ADDR_SPACE_BITS 36
+#define TARGET_VIRT_ADDR_SPACE_BITS 32
+#endif
+
But I really don't understand why it was changed to 52 when the
macros were moved. A (fortunate) typo?
--
Eduardo
