Re: [PATCH v2 3/3] kvm: svm: Use the hardware provided GPA instead of page walk

From: Brijesh Singh
Date: Wed Dec 14 2016 - 13:55:13 EST

On 12/14/2016 11:23 AM, Paolo Bonzini wrote:

On 14/12/2016 18:07, Brijesh Singh wrote:

Since now we are going to perform multiple conditional checks before
concluding that its safe to use HW provided GPA. How about if we add two
functions "emulator_is_rep_string_op" and "emulator_is_two_mem_op" into
emulator.c and use these functions inside the x86.c to determine if its
safe to use HW provided gpa?

Why not export only emulator_can_use_gpa from emulate.c? (So in the end
leaving emulator_is_string_op in emulate.c was the right thing to do, it
was just the test that was wrong :)).

Actually, I was not sure if putting emulator_can_use_gpa() in emulate.c was right thing - mainly because emulator.c does not deal with GPA. I will go with your advice and put it in emulator.c, it makes easy :)

The patch below is still missing the check for cross-page MMIO. Your
reference to the BKDG only covers MMCONFIG (sometimes referred to as
ECAM), not MMIO in general. Doing AND or OR into video memory for
example is perfectly legal, and I'm fairly sure that some obscure legacy
software does PUSH/POP into vram as well!

I used your below code snippet to detect cross-page MMIO access. After applying these changes cross-page MMIO read/write unit test is passing just fine. I will include it in patch.

> Actually there is a nice trick you can do to support cross-page
> MMIO access detection:

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 37cd31645d45..754d251dc611 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4549,6 +4549,7 @@ static int emulator_read_write_onepage(unsigned long addr, void *val,
if (vcpu->arch.gpa_available &&
!emulator_can_use_hw_gpa(ctxt) &&
+ (addr & ~PAGE_MASK) == (exception->address & ~PAGE_MASK) &&
vcpu_is_mmio_gpa(vcpu, addr, exception->address, write)) {
gpa = exception->address;
goto mmio;