[PATCH 2/2] drivers/hv: correct tsc page sequence invalid value

From: K. Y. Srinivasan
Date: Wed Nov 18 2015 - 14:34:01 EST

From: Andrey Smetanin <asmetanin@xxxxxxxxxxxxx>

Hypervisor Top Level Functional Specification v3/4 says
that TSC page sequence value = -1(0xFFFFFFFF) is used to
indicate that TSC page no longer reliable source of reference
timer. Unfortunately, we found that Windows Hyper-V guest
side implementation uses sequence value = 0 to indicate
that Tsc page no longer valid. This is clearly visible
inside Windows 2012R2 ntoskrnl.exe HvlGetReferenceTime()
function dissassembly:

HvlGetReferenceTime proc near
xchg ax, ax
mov rax, cs:HvlpReferenceTscPage
mov r9d, [rax]
test r9d, r9d
jz short loc_1401C3176
mov rcx, cs:HvlpReferenceTscPage
shl rdx, 20h
or rdx, rax
mov rax, [rcx+8]
mov rcx, cs:HvlpReferenceTscPage
mov r8, [rcx+10h]
mul rdx
mov rax, cs:HvlpReferenceTscPage
add rdx, r8
mov ecx, [rax]
cmp ecx, r9d
jnz short loc_1401C3132
jmp short loc_1401C3184
mov ecx, 40000020h
shl rdx, 20h
or rdx, rax
mov rax, rdx
HvlGetReferenceTime endp

This patch aligns Tsc page invalid sequence value with
Windows Hyper-V guest implementation which is more
compatible with both Hyper-V hypervisor and KVM hypervisor.

Signed-off-by: Andrey Smetanin <asmetanin@xxxxxxxxxxxxx>
Signed-off-by: Denis V. Lunev <den@xxxxxxxxxx>
CC: "K. Y. Srinivasan" <kys@xxxxxxxxxxxxx>
CC: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
CC: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>

Signed-off-by: Denis V. Lunev <den@xxxxxxxxxx>
Signed-off-by: K. Y. Srinivasan <kys@xxxxxxxxxxxxx>
drivers/hv/hv.c | 4 ++--
1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/hv/hv.c b/drivers/hv/hv.c
index 7a06933..1db9556 100644
--- a/drivers/hv/hv.c
+++ b/drivers/hv/hv.c
@@ -140,7 +140,7 @@ static cycle_t read_hv_clock_tsc(struct clocksource *arg)
cycle_t current_tick;
struct ms_hyperv_tsc_page *tsc_pg = hv_context.tsc_page;

- if (tsc_pg->tsc_sequence != -1) {
+ if (tsc_pg->tsc_sequence != 0) {
* Use the tsc page to compute the value.
@@ -162,7 +162,7 @@ static cycle_t read_hv_clock_tsc(struct clocksource *arg)
if (tsc_pg->tsc_sequence == sequence)
return current_tick;

- if (tsc_pg->tsc_sequence != -1)
+ if (tsc_pg->tsc_sequence != 0)
* Fallback using MSR method.

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