Re: [PATCH v2 07/25] KVM: arm64: vgic-v3: Add ICV_IAR1_EL1 handler

Christoffer Dall
2017-06-06T13:46:26Z

On Tue, Jun 06, 2017 at 02:35:30PM +0100, Marc Zyngier wrote:
> On 06/06/17 12:09, Christoffer Dall wrote:
> > On Mon, Jun 05, 2017 at 11:33:52AM +0100, Marc Zyngier wrote:
> >> On 05/06/17 10:21, Christoffer Dall wrote:
> >>> On Thu, Jun 01, 2017 at 11:20:59AM +0100, Marc Zyngier wrote:
> >>>> Add a handler for reading the guest's view of the ICC_IAR1_EL1
> >>>> register. This involves finding the highest priority Group-1
> >>>> interrupt, checking against both PMR and the active group
> >>>> priority, activating the interrupt and setting the group
> >>>> priority as active.
> >>>>
> >>>> Reviewed-by: Eric Auger <eric.auger@redhat.com>
> >>>> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> >>>> ---
> >>>> include/linux/irqchip/arm-gic-v3.h | 1 +
> >>>> virt/kvm/arm/hyp/vgic-v3-sr.c | 150 +++++++++++++++++++++++++++++++++++++
> >>>> 2 files changed, 151 insertions(+)
> >>>>
> >>>> diff --git a/include/linux/irqchip/arm-gic-v3.h b/include/linux/irqchip/arm-gic-v3.h
> >>>> index fffb91202bc9..401db585a534 100644
> >>>> --- a/include/linux/irqchip/arm-gic-v3.h
> >>>> +++ b/include/linux/irqchip/arm-gic-v3.h
> >>>> @@ -405,6 +405,7 @@
> >>>> #define ICH_LR_PHYS_ID_SHIFT 32
> >>>> #define ICH_LR_PHYS_ID_MASK (0x3ffULL << ICH_LR_PHYS_ID_SHIFT)
> >>>> #define ICH_LR_PRIORITY_SHIFT 48
> >>>> +#define ICH_LR_PRIORITY_MASK (0xffULL << ICH_LR_PRIORITY_SHIFT)
> >>>>
> >>>> /* These are for GICv2 emulation only */
> >>>> #define GICH_LR_VIRTUALID (0x3ffUL << 0)
> >>>> diff --git a/virt/kvm/arm/hyp/vgic-v3-sr.c b/virt/kvm/arm/hyp/vgic-v3-sr.c
> >>>> index 168539dfd0b9..16a2eadc7a5c 100644
> >>>> --- a/virt/kvm/arm/hyp/vgic-v3-sr.c
> >>>> +++ b/virt/kvm/arm/hyp/vgic-v3-sr.c
> >>>> @@ -24,6 +24,7 @@
> >>>>
> >>>> #define vtr_to_max_lr_idx(v) ((v) & 0xf)
> >>>> #define vtr_to_nr_pre_bits(v) (((u32)(v) >> 26) + 1)
> >>>> +#define vtr_to_nr_apr_regs(v) (1 << (vtr_to_nr_pre_bits(v) - 5))
> >>>>
> >>>> static u64 __hyp_text __gic_v3_get_lr(unsigned int lr)
> >>>> {
> >>>> @@ -375,6 +376,79 @@ void __hyp_text __vgic_v3_write_vmcr(u32 vmcr)
> >>>>
> >>>> #ifdef CONFIG_ARM64
> >>>>
> >>>> +static int __hyp_text __vgic_v3_get_group(struct kvm_vcpu *vcpu)
> >>>> +{
> >>>> + u32 esr = kvm_vcpu_get_hsr(vcpu);
> >>>> + u8 crm = (esr & ESR_ELx_SYS64_ISS_CRM_MASK) >> ESR_ELx_SYS64_ISS_CRM_SHIFT;
> >>>> +
> >>>> + return crm != 8;
> >>>> +}
> >>>> +
> >>>> +#define GICv3_IDLE_PRIORITY 0xff
> >>>> +
> >>>> +static int __hyp_text __vgic_v3_highest_priority_lr(struct kvm_vcpu *vcpu,
> >>>> + u32 vmcr,
> >>>> + u64 *lr_val)
> >>>> +{
> >>>> + unsigned int used_lrs = vcpu->arch.vgic_cpu.used_lrs;
> >>>> + u8 priority = GICv3_IDLE_PRIORITY;
> >>>> + int i, lr = -1;
> >>>> +
> >>>> + for (i = 0; i < used_lrs; i++) {
> >>>> + u64 val = __gic_v3_get_lr(i);
> >>>> + u8 lr_prio = (val & ICH_LR_PRIORITY_MASK) >> ICH_LR_PRIORITY_SHIFT;
> >>>> +
> >>>> + /* Not pending in the state? */
> >>>> + if ((val & ICH_LR_STATE) != ICH_LR_PENDING_BIT)
> >>>> + continue;
> >>>> +
> >>>> + /* Group-0 interrupt, but Group-0 disabled? */
> >>>> + if (!(val & ICH_LR_GROUP) && !(vmcr & ICH_VMCR_ENG0_MASK))
> >>>> + continue;
> >>>> +
> >>>> + /* Group-1 interrupt, but Group-1 disabled? */
> >>>> + if ((val & ICH_LR_GROUP) && !(vmcr & ICH_VMCR_ENG1_MASK))
> >>>> + continue;
> >>>> +
> >>>> + /* Not the highest priority? */
> >>>> + if (lr_prio >= priority)
> >>>> + continue;
> >>>> +
> >>>> + /* This is a candidate */
> >>>> + priority = lr_prio;
> >>>> + *lr_val = val;
> >>>> + lr = i;
> >>>> + }
> >>>> +
> >>>> + if (lr == -1)
> >>>> + *lr_val = ICC_IAR1_EL1_SPURIOUS;
> >>>> +
> >>>> + return lr;
> >>>> +}
> >>>> +
> >>>> +static int __hyp_text __vgic_v3_get_highest_active_priority(void)
> >>>> +{
> >>>> + u8 nr_pre_bits = vtr_to_nr_pre_bits(read_gicreg(ICH_VTR_EL2));
> >>>> + u8 nr_apr_regs = vtr_to_nr_apr_regs(read_gicreg(ICH_VTR_EL2));
> >>>> + u32 hap = 0;
> >>>> + int i;
> >>>> +
> >>>> + for (i = 0; i < nr_apr_regs; i++) {
> >>>> + u32 val;
> >>>> +
> >>>> + val = __vgic_v3_read_ap0rn(i);
> >>>> + val |= __vgic_v3_read_ap1rn(i);
> >>>> + if (!val) {
> >>>> + hap += 32;
> >>>> + continue;
> >>>> + }
> >>>> +
> >>>> + return (hap + __ffs(val)) << (8 - nr_pre_bits);
> >>>
> >>> I don't understand this shift, and I think I asked about it before, so
> >>> maybe if it's a reused concept we can use a static inline or at least
> >>> provide a comment?
> >>
> >> I tried to explain it in my reply to your comment on patch #5. Can
> >> definitely make that a helper. I think part of the confusion is that
> >> this constant is used in a number of ways to express the conversion
> >> between a preemption level and a priority.
> >>
> >
> > ok, I understand it now, but it's weird that we use 8 as a constant here
> > (which applied to both group 0 and group 1) but 7 and 8, respecitvely,
> > for the writes to the bpr0.
>
> I made it slightly simpler to understand in my current series (hopefully).
>
> > I understand that these two concepts are actually independent and
> > loosely related, so maybe adding something like this would help:
> >
> > /*
> > * The ICH_AP0Rn_EL2 and ICH_AP1Rn_EL2 registers contain
> > * the active priority levels for this VCPU for the
> > * maximum number of supported priority levels, and we
> > * return the full priority level only if the BPR is
> > * programmed to its minimum, otherwise we return a
> > * combination of the priority level and subpriority, as
> > * determined by the setting of the BPR, but without the
> > * full subpriority.
> > */
> >
> > Maybe this is wrong and will just confuse people more?
>
> It seems right to me, so I'll paste it in.
>
> >>>> + }
> >>>> +
> >>>> + return GICv3_IDLE_PRIORITY;
> >>>> +}
> >>>> +
> >>>> static unsigned int __hyp_text __vgic_v3_get_bpr0(u32 vmcr)
> >>>> {
> >>>> return (vmcr & ICH_VMCR_BPR0_MASK) >> ICH_VMCR_BPR0_SHIFT;
> >>>> @@ -395,6 +469,79 @@ static unsigned int __hyp_text __vgic_v3_get_bpr1(u32 vmcr)
> >>>> return bpr;
> >>>> }
> >>>>
> >>>> +/*
> >>>> + * Convert a priority to a preemption level, taking the relevant BPR
> >>>> + * into account by zeroing the sub-priority bits.
> >>>> + */
> >>>> +static u8 __hyp_text __vgic_v3_pri_to_pre(u8 pri, u32 vmcr, int grp)
> >>>> +{
> >>>> + unsigned int bpr;
> >>>> +
> >>>> + if (!grp)
> >>>> + bpr = __vgic_v3_get_bpr0(vmcr) + 1;
> >>>> + else
> >>>> + bpr = __vgic_v3_get_bpr1(vmcr);
> >>>> +
> >>>> + return pri & (GENMASK(7, 0) << bpr);
> >>>> +}
> >>>> +
> >>>> +/*
> >>>> + * The priority value is independent of any of the BPR values, so we
> >>>> + * normalize it using nr_pre_bits. This guarantees that no matter what
> >>>> + * the guest does with its BPR, we can always set/get the same value
> >>>> + * of a priority.
> >>>> + */
> >>>> +static void __hyp_text __vgic_v3_set_active_priority(u8 pri, u32 vmcr, int grp)
> >>>> +{
> >>>> + u8 nr_pre_bits, pre, ap;
> >>>> + u32 val;
> >>>> + int apr;
> >>>> +
> >>>> + nr_pre_bits = vtr_to_nr_pre_bits(read_gicreg(ICH_VTR_EL2));
> >>>> + pre = __vgic_v3_pri_to_pre(pri, vmcr, grp);
> >>>> + ap = pre >> (8 - nr_pre_bits);
> >>>
> >>> Again here, I don't get this shift.
> >>>
> >>>> + apr = ap / 32;
> >>>> +
> >>>> + val = __vgic_v3_read_ap1rn(apr);
> >>>> + __vgic_v3_write_ap1rn(val | BIT(ap % 32), apr);
> >>>
> >>> How are we sure this is a group 1 interrupt here?
> >>
> >> That's a bug, as we should definitely check grp here. Thanks for
> >> noticing it!
> >>
> >
> > Sure. Also, the spec says "Writing to these registers with any value
> > other than the last read value of the register (or 0x00000000 for a
> > newly set up virtual machine) can result in UNPREDICTABLE behavior of
> > the virtual interrupt prioritization system allowing either: ...". Does
> > that just translate to "You should know what you're doing", or could we
> > be breaking something here?
>
> This is basically a "don't mess with the HW". But in our case, we're mostly
> using the registers as storage, and doing the heavy lifting in SW.
>
> >
> >>>> +}
> >>>> +
> >>>> +static void __hyp_text __vgic_v3_read_iar(struct kvm_vcpu *vcpu, u32 vmcr, int rt)
> >>>> +{
> >>>> + u64 lr_val;
> >>>> + u8 lr_prio, pmr;
> >>>> + int lr, grp;
> >>>> +
> >>>> + grp = __vgic_v3_get_group(vcpu);
> >>>> +
> >>>> + lr = __vgic_v3_highest_priority_lr(vcpu, vmcr, &lr_val);
> >>>> + if (lr < 0)
> >>>> + goto spurious;
> >>>> +
> >>>> + if (grp != !!(lr_val & ICH_LR_GROUP))
> >>>> + goto spurious;
> >>>> +
> >>>> + pmr = (vmcr & ICH_VMCR_PMR_MASK) >> ICH_VMCR_PMR_SHIFT;
> >>>> + lr_prio = (lr_val & ICH_LR_PRIORITY_MASK) >> ICH_LR_PRIORITY_SHIFT;
> >>>> + if (pmr <= lr_prio)
> >>>> + goto spurious;
> >>>> +
> >>>> + if (__vgic_v3_get_highest_active_priority() <= lr_prio)
> >>>> + goto spurious;
> >
> > Based on what I wrote above, don't you need to consider the actual
> > setting of the BPR here?
> >
> > For example, if __vgic_v3_get_highest_active_priority() == 100.10 and
> > lr_prio == 100.09 (BPR == 5 for a group 1 interrupt) then you'll
> > compare 10010 <= 10009, you won't take the branch and you'll let the
> > guest ack another interrupt at the same-and-already-active preemption
> > level.
> >
> > Or am I again misunderstandin how this whole thing works?
>
> I think you're right. This should read:
>
> diff --git a/virt/kvm/arm/hyp/vgic-v3-sr.c b/virt/kvm/arm/hyp/vgic-v3-sr.c
> index 78e267637bbd..8f565983de08 100644
> --- a/virt/kvm/arm/hyp/vgic-v3-sr.c
> +++ b/virt/kvm/arm/hyp/vgic-v3-sr.c
> @@ -604,7 +604,7 @@ static void __hyp_text __vgic_v3_read_iar(struct kvm_vcpu *vcpu, u32 vmcr, int r
> if (pmr <= lr_prio)
> goto spurious;
>
> - if (__vgic_v3_get_highest_active_priority() <= lr_prio)
> + if (__vgic_v3_get_highest_active_priority() <= __vgic_v3_pri_to_pre(lr_prio, vmcr, grp))
> goto spurious;
>
> lr_val &= ~ICH_LR_STATE;
>
> so that we actually compare the preemption levels.
>

That looks good.

> >
> >>>> +
> >>>> + lr_val &= ~ICH_LR_STATE;
> >>>> + /* No active state for LPIs */
> >>>> + if ((lr_val & ICH_LR_VIRTUAL_ID_MASK) <= VGIC_MAX_SPI)
> >>>> + lr_val |= ICH_LR_ACTIVE_BIT;
> >>>> + __gic_v3_set_lr(lr_val, lr);
> >>>> + __vgic_v3_set_active_priority(lr_prio, vmcr, grp);
> >>>> + vcpu_set_reg(vcpu, rt, lr_val & ICH_LR_VIRTUAL_ID_MASK);
> >>>> + return;
> >>>> +
> >>>> +spurious:
> >>>> + vcpu_set_reg(vcpu, rt, ICC_IAR1_EL1_SPURIOUS);
> >>>> +}
> >>>> +
> >>>> static void __hyp_text __vgic_v3_read_igrpen1(struct kvm_vcpu *vcpu, u32 vmcr, int rt)
> >>>> {
> >>>> vcpu_set_reg(vcpu, rt, !!(vmcr & ICH_VMCR_ENG1_MASK));
> >>>> @@ -459,6 +606,9 @@ int __hyp_text __vgic_v3_perform_cpuif_access(struct kvm_vcpu *vcpu)
> >>>> is_read = (esr & ESR_ELx_SYS64_ISS_DIR_MASK) == ESR_ELx_SYS64_ISS_DIR_READ;
> >>>>
> >>>> switch (sysreg) {
> >>>> + case SYS_ICC_IAR1_EL1:
> >>>> + fn = __vgic_v3_read_iar;
> >>>> + break;
> >>>
> >>> So we don't provide a write-ignore function here because we rely on the
> >>> hardware to always trap that at EL1 instead?
> >>>
> >>> I remember we discussed this before, but I don't remember if the
> >>> conclusion was that this is 100% safe.
> >>
> >> A properly designed CPU would UNDEF at EL1. I'm happy to try and detect
> >> broken failing implementations here, but I'm not sure of what to do, as
> >> we're not in the best context to handle this. We could continue exiting
> >> to EL1 and handle things there...
> >>
> >
> > I feel like we decided that other parts of KVM relies on this being
> > implemented correctly, so maybe this is not the place to begin being
> > overly cautious about things.
> >
> > That said, having a path back to EL1 where we can print stuff and
> > create warnings, may not be an entirely bad idea.
>
> I'll look at adding something that takes care of it.
>

Thanks!
-Christoffer