[PATCH 1/1] drivers/hv: correct tsc page sequence invalid value

[Denis V. Lunev]

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that Tsc page no longer valid.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
loc_1401C3132:
                 mov     rax, cs:HvlpReferenceTscPage
                 mov     r9d, [rax]
                 test    r9d, r9d
                 jz      short loc_1401C3176
                 rdtsc
                 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
loc_1401C3176:
                 mov     ecx, 40000020h
                 rdmsr
                 shl     rdx, 20h
                 or      rdx, rax
loc_1401C3184:
                 mov     rax, rdx
                 retn
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>
CC: "K. Y. Srinivasan" <kys@xxxxxxxxxxxxx>
CC: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
CC: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>

TARGET: Microsoft Hyper-V Linux guest drivers
Signed-off-by: Denis V. Lunev <den@xxxxxxxxxx>
---
 drivers/hv/hv.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/hv/hv.c b/drivers/hv/hv.c
index 6341be8..68d0701 100644
--- a/drivers/hv/hv.c
+++ b/drivers/hv/hv.c
@@ -139,7 +139,7 @@ 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.
 		 */
@@ -161,7 +161,7 @@ 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)
 				continue;
 			/*
 			 * Fallback using MSR method.
-- 
2.1.4

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