On 15/06/2022 11:36, Wei Chen wrote:
Hi Julien,
Hi Wei,
-----Original Message-----
From: Julien Grall <jul...@xen.org>
Sent: 2022年6月15日 17:47
To: Wei Chen <wei.c...@arm.com>; xen-devel@lists.xenproject.org
Cc: nd <n...@arm.com>; Stefano Stabellini <sstabell...@kernel.org>; Bertrand
Marquis <bertrand.marq...@arm.com>; Volodymyr Babchuk
<volodymyr_babc...@epam.com>
Subject: Re: [PATCH] xen/arm: avoid vtimer flip-flop transition in context
switch
So in this patch, we adjust the formula to use "offset - boot_count"
first, and then use the result to plus cval. This will avoid the
uint64_t overflow.
Technically, the overflow is still present because the (offset -
boot_count) is a non-zero value *and* cval is a 64-bit value.
Yes, GuestOS can issue any valid 64-bit value for their usage.
So I think the equation below should be reworked to...
Signed-off-by: Wei Chen <wei.c...@arm.com>
---
xen/arch/arm/vtimer.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/xen/arch/arm/vtimer.c b/xen/arch/arm/vtimer.c
index 5bb5970f58..86e63303c8 100644
--- a/xen/arch/arm/vtimer.c
+++ b/xen/arch/arm/vtimer.c
@@ -144,8 +144,9 @@ void virt_timer_save(struct vcpu *v)
if ( (v->arch.virt_timer.ctl & CNTx_CTL_ENABLE) &&
!(v->arch.virt_timer.ctl & CNTx_CTL_MASK))
{
- set_timer(&v->arch.virt_timer.timer, ticks_to_ns(v-
arch.virt_timer.cval +
- v->domain->arch.virt_timer_base.offset - boot_count));
+ set_timer(&v->arch.virt_timer.timer,
+ ticks_to_ns(v->domain->arch.virt_timer_base.offset -
+ boot_count + v->arch.virt_timer.cval));
... something like:
ticks_to_ns(offset - boot_count) + ticks_to_ns(cval);
The first part of the equation should always be the same. So it could be
stored in struct domain.
If you think there is still some values to continue this patch, I will
address this comment : )
I think there are. This can be easily triggered by a vCPU setting a
large cval.
Cheers,
--
Julien Grall
