> -----Original Message----- > From: Will Deacon [mailto:will@xxxxxxxxxx] > Sent: 21 July 2021 17:06 > To: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@xxxxxxxxxx> > Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; kvmarm@xxxxxxxxxxxxxxxxxxxxx; > linux-kernel@xxxxxxxxxxxxxxx; maz@xxxxxxxxxx; catalin.marinas@xxxxxxx; > james.morse@xxxxxxx; julien.thierry.kdev@xxxxxxxxx; > suzuki.poulose@xxxxxxx; jean-philippe@xxxxxxxxxx; > Alexandru.Elisei@xxxxxxx; Linuxarm <linuxarm@xxxxxxxxxx> > Subject: Re: [PATCH v2 2/3] kvm/arm: Introduce a new vmid allocator for KVM > > On Wed, Jun 16, 2021 at 04:56:05PM +0100, Shameer Kolothum wrote: > > A new VMID allocator for arm64 KVM use. This is based on > > arm64 asid allocator algorithm. > > > > Signed-off-by: Shameer Kolothum > <shameerali.kolothum.thodi@xxxxxxxxxx> > > --- > > arch/arm64/include/asm/kvm_host.h | 4 + > > arch/arm64/kvm/vmid.c | 206 > ++++++++++++++++++++++++++++++ > > 2 files changed, 210 insertions(+) > > create mode 100644 arch/arm64/kvm/vmid.c > > Generally, I prefer this to the alternative of creating a library. However, > I'd probably remove all the duplicated comments in favour of a reference > to the ASID allocator. That way, we can just comment any VMID-specific > behaviour in here. Agree. I retained the comments mainly for myself as its very difficult at times to follow :) > > Some comments below... > > > diff --git a/arch/arm64/include/asm/kvm_host.h > b/arch/arm64/include/asm/kvm_host.h > > index 7cd7d5c8c4bc..75a7e8071012 100644 > > --- a/arch/arm64/include/asm/kvm_host.h > > +++ b/arch/arm64/include/asm/kvm_host.h > > @@ -680,6 +680,10 @@ int kvm_arm_pvtime_get_attr(struct kvm_vcpu > *vcpu, > > int kvm_arm_pvtime_has_attr(struct kvm_vcpu *vcpu, > > struct kvm_device_attr *attr); > > > > +int kvm_arm_vmid_alloc_init(void); > > +void kvm_arm_vmid_alloc_free(void); > > +void kvm_arm_update_vmid(atomic64_t *id); > > + > > static inline void kvm_arm_pvtime_vcpu_init(struct kvm_vcpu_arch > *vcpu_arch) > > { > > vcpu_arch->steal.base = GPA_INVALID; > > diff --git a/arch/arm64/kvm/vmid.c b/arch/arm64/kvm/vmid.c > > new file mode 100644 > > index 000000000000..687e18d33130 > > --- /dev/null > > +++ b/arch/arm64/kvm/vmid.c > > @@ -0,0 +1,206 @@ > > +// SPDX-License-Identifier: GPL-2.0 > > +/* > > + * VMID allocator. > > + * > > + * Based on arch/arm64/mm/context.c > > + * > > + * Copyright (C) 2002-2003 Deep Blue Solutions Ltd, all rights reserved. > > + * Copyright (C) 2012 ARM Ltd. > > + */ > > + > > +#include <linux/bitfield.h> > > +#include <linux/bitops.h> > > + > > +#include <asm/kvm_asm.h> > > +#include <asm/kvm_mmu.h> > > + > > +static u32 vmid_bits; > > +static DEFINE_RAW_SPINLOCK(cpu_vmid_lock); > > + > > +static atomic64_t vmid_generation; > > +static unsigned long *vmid_map; > > + > > +static DEFINE_PER_CPU(atomic64_t, active_vmids); > > +static DEFINE_PER_CPU(u64, reserved_vmids); > > +static cpumask_t tlb_flush_pending; > > + > > +#define VMID_MASK (~GENMASK(vmid_bits - 1, 0)) > > +#define VMID_FIRST_VERSION (1UL << vmid_bits) > > + > > +#define NUM_USER_VMIDS VMID_FIRST_VERSION > > +#define vmid2idx(vmid) ((vmid) & ~VMID_MASK) > > +#define idx2vmid(idx) vmid2idx(idx) > > + > > +#define vmid_gen_match(vmid) \ > > + (!(((vmid) ^ atomic64_read(&vmid_generation)) >> vmid_bits)) > > + > > +static void flush_context(void) > > +{ > > + int cpu; > > + u64 vmid; > > + > > + bitmap_clear(vmid_map, 0, NUM_USER_VMIDS); > > + > > + for_each_possible_cpu(cpu) { > > + vmid = atomic64_xchg_relaxed(&per_cpu(active_vmids, cpu), 0); > > + /* > > + * If this CPU has already been through a > > + * rollover, but hasn't run another task in > > + * the meantime, we must preserve its reserved > > + * VMID, as this is the only trace we have of > > + * the process it is still running. > > + */ > > + if (vmid == 0) > > + vmid = per_cpu(reserved_vmids, cpu); > > + __set_bit(vmid2idx(vmid), vmid_map); > > + per_cpu(reserved_vmids, cpu) = vmid; > > + } > > Hmm, so here we're copying the active_vmids into the reserved_vmids on a > rollover, but I wonder if that's overly pessismistic? For the ASID > allocator, every CPU tends to have a current task so it makes sense, but > I'm not sure it's necessarily the case that every CPU tends to have a > vCPU as the current task. For example, imagine you have a nasty 128-CPU > system with 8-bit VMIDs and each CPU has at some point run a vCPU. Then, > on rollover, we'll immediately reserve half of the VMID space, even if > those vCPUs don't even exist any more. > > Not sure if it's worth worrying about, but I wanted to mention it. Ok. I see your suggestion in patch #3 to avoid this. > > > +void kvm_arm_update_vmid(atomic64_t *id) > > +{ > > Take the kvm_vmid here? That would make: > > > + /* Check that our VMID belongs to the current generation. */ > > + vmid = atomic64_read(id); > > + if (!vmid_gen_match(vmid)) { > > + vmid = new_vmid(id); > > + atomic64_set(id, vmid); > > + } > > A bit more readable, as you could pass the pointer directly to new_vmid > for initialisation. Ok. Thanks, Shameer _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
