Re: [PATCH 23/23] KVM: arm64: Treat emulated TVAL TimerValue as a signed 32-bit integer

[Marc Zyngier <maz@xxxxxxxxxx>]

On 2020-01-30 14:11, Alexandru Elisei wrote:
> Hi,
>
> On 1/30/20 1:25 PM, Marc Zyngier wrote:
> > From: Alexandru Elisei <alexandru.elisei@xxxxxxx>
> >
> > According to the ARM ARM, registers CNT{P,V}_TVAL_EL0 have bits 
> > [63:32]
> > RES0 [1]. When reading the register, the value is truncated to the 
> > least
> > significant 32 bits [2], and on writes, TimerValue is treated as a 
> > signed
> > 32-bit integer [1, 2].
> >
> > When the guest behaves correctly and writes 32-bit values, treating 
> > TVAL
> > as an unsigned 64 bit register works as expected. However, things 
> > start
> > to break down when the guest writes larger values, because
> > (u64)0x1_ffff_ffff = 8589934591. but (s32)0x1_ffff_ffff = -1, and the
> > former will cause the timer interrupt to be asserted in the future, 
> > but
> > the latter will cause it to be asserted now.  Let's treat TVAL as a
> > signed 32-bit register on writes, to match the behaviour described in
> > the architecture, and the behaviour experimentally exhibited by the
> > virtual timer on a non-vhe host.
> >
> > [1] Arm DDI 0487E.a, section D13.8.18
> > [2] Arm DDI 0487E.a, section D11.2.4
> >
> > Signed-off-by: Alexandru Elisei <alexandru.elisei@xxxxxxx>
> > [maz: replaced the read-side mask with lower_32_bits]
> > Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
> > Fixes: 8fa761624871 ("KVM: arm/arm64: arch_timer: Fix CNTP_TVAL 
> > calculation")
> > Link: 
> > https://lore.kernel.org/r/20200127103652.2326-1-alexandru.elisei@xxxxxxx
> > ---
> >  virt/kvm/arm/arch_timer.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> > index f182b2380345..c6c2a9dde00c 100644
> > --- a/virt/kvm/arm/arch_timer.c
> > +++ b/virt/kvm/arm/arch_timer.c
> > @@ -805,6 +805,7 @@ static u64 kvm_arm_timer_read(struct kvm_vcpu 
> > *vcpu,
> >  	switch (treg) {
> >  	case TIMER_REG_TVAL:
> >  		val = timer->cnt_cval - kvm_phys_timer_read() + timer->cntvoff;
> > +		val &= lower_32_bits(val);
>
> This is correct, but how about making it val = lower_32_bits(val) for 
> more
> clarity? Apologies for not spotting it earlier :(

That's what it should have been, but I obviously typoed it. As it passed
all my tests, I didn't notice the issue. I'll queue a cleanup once Paolo
has a chance to pull this.

        M.
--
Jazz is not dead. It just smells funny...