On 04/07/2011 01:18 PM, Jordan Justen wrote:
On Thu, Apr 7, 2011 at 09:08, Anthony Liguori<anthony@xxxxxxxxxxxxx> wrote:
On 04/07/2011 10:51 AM, Gleb Natapov wrote:
That may seams to be impossible but it is how HW works. And this is how
QEMU emulates it. Look at target-i386/helper.c:cpu_reset()
cpu_x86_load_seg_cache(env, R_CS, 0xf000, 0xffff0000, 0xffff,
DESC_P_MASK | DESC_S_MASK | DESC_CS_MASK |
DESC_R_MASK | DESC_A_MASK);
env->eip = 0xfff0;
Don't know how a20 gate is handled btw.
I see that we use 0xf0000 in the kernel but this is because of a limitation
of VMX.
I recently noticed that kvm does this. It does not seem to be a big
deal as firmware can easily deal with it, but I did find it odd that
kvm had the csbase of 0xf0000 as processors generally use a csbase of
0xffff0000 initially. (At least, this is what I've seen with Intel
processors for the past 12 years.)
How can this limitation exist with VMX if mode transitions are
supported, in which case this type of csbase vs. real-mode segment
mismatch can easily occur?
Mismatches between cached segment descriptors and the segment registers
are not problematic in KVM per say.
The issue is that vm8086 mode doesn't allow for this and since KVM uses
vm8086 to emulate real mode on processors that don't support whatever it
is that Intel is calling it these days, we can't effectively support this.
I guess when 32-bit was introduced, this behavior was added.
The CS base starts out at 0xf0000 and IP is 0xfff0. That gives a
real address of 0xffff0. This is usually a trampoline to somewhere
else in the space.
CS descriptor and CS selector don't have to be in sync (big real mode).
Indeed.
Another place this will often be seen is SMM, as the SMBASE can easily
be> 1MB, but the SMM entry is in 16 bit mode.
KVM doesn't support SMM although that's not because of this. KVM
doesn't allow execution of ROM memory which makes it difficult to
implement PAM in the way it's intended to be implemented. This makes
SMM a bit tricky to make work. Since there's never really been a
pressing need to support SMM, to my knowledge, noone has even tried.
Regards,
Anthony Liguori
-Jordan
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