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Date: Mon, 15 Jul 2019 18:06:28 +0100 From: Julien Grall <julien.grall@....com> To: James Morse <james.morse@....com> Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu, marc.zyngier@....com, julien.thierry@....com, suzuki.poulose@....com, catalin.marinas@....com, will.deacon@....com Subject: Re: [RFC v2 14/14] kvm/arm: Align the VMID allocation with the arm64 ASID one On 03/07/2019 18:36, James Morse wrote: > Hi Julien, Hi James, > On 20/06/2019 14:06, Julien Grall wrote: >> At the moment, the VMID algorithm will send an SGI to all the CPUs to >> force an exit and then broadcast a full TLB flush and I-Cache >> invalidation. >> >> This patch re-use the new ASID allocator. The >> benefits are: >> - CPUs are not forced to exit at roll-over. Instead the VMID will be >> marked reserved and the context will be flushed at next exit. This >> will reduce the IPIs traffic. >> - Context invalidation is now per-CPU rather than broadcasted. > > + Catalin has a model of the asid-allocator. That's a good point :). > > >> With the new algo, the code is now adapted: >> - The function __kvm_flush_vm_context() has been renamed to >> __kvm_flush_cpu_vmid_context and now only flushing the current CPU context. >> - The call to update_vttbr() will be done with preemption disabled >> as the new algo requires to store information per-CPU. >> - The TLBs associated to EL1 will be flushed when booting a CPU to >> deal with stale information. This was previously done on the >> allocation of the first VMID of a new generation. >> >> The measurement was made on a Seattle based SoC (8 CPUs), with the >> number of VMID limited to 4-bit. The test involves running concurrently 40 >> guests with 2 vCPUs. Each guest will then execute hackbench 5 times >> before exiting. > >> diff --git a/arch/arm64/include/asm/kvm_asid.h b/arch/arm64/include/asm/kvm_asid.h >> new file mode 100644 >> index 000000000000..8b586e43c094 >> --- /dev/null >> +++ b/arch/arm64/include/asm/kvm_asid.h >> @@ -0,0 +1,8 @@ >> +/* SPDX-License-Identifier: GPL-2.0 */ >> +#ifndef __ARM64_KVM_ASID_H__ >> +#define __ARM64_KVM_ASID_H__ >> + >> +#include <asm/asid.h> >> + >> +#endif /* __ARM64_KVM_ASID_H__ */ >> + >> diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h >> index ff73f5462aca..06821f548c0f 100644 >> --- a/arch/arm64/include/asm/kvm_asm.h >> +++ b/arch/arm64/include/asm/kvm_asm.h >> @@ -62,7 +62,7 @@ extern char __kvm_hyp_init_end[]; >> >> extern char __kvm_hyp_vector[]; >> >> -extern void __kvm_flush_vm_context(void); >> +extern void __kvm_flush_cpu_vmid_context(void); >> extern void __kvm_tlb_flush_vmid_ipa(struct kvm *kvm, phys_addr_t ipa); > > As we've got a __kvm_tlb_flush_local_vmid(), would __kvm_tlb_flush_local_all() fit in > better? (This mirrors local_flush_tlb_all() too) I am happy with the renaming here. > > >> extern void __kvm_tlb_flush_vmid(struct kvm *kvm); >> extern void __kvm_tlb_flush_local_vmid(struct kvm_vcpu *vcpu); >> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h >> index 4bcd9c1291d5..7ef45b7da4eb 100644 >> --- a/arch/arm64/include/asm/kvm_host.h >> +++ b/arch/arm64/include/asm/kvm_host.h >> @@ -68,8 +68,8 @@ int kvm_arch_vm_ioctl_check_extension(struct kvm *kvm, long ext); >> void __extended_idmap_trampoline(phys_addr_t boot_pgd, phys_addr_t idmap_start); >> >> struct kvm_vmid { >> - /* The VMID generation used for the virt. memory system */ >> - u64 vmid_gen; >> + /* The ASID used for the ASID allocator */ >> + atomic64_t asid; > > Can we call this 'id' as happens in mm_context_t? (calling it asid is confusing) I am fine with this suggestion. > >> u32 vmid; > > Can we filter out the generation bits in kvm_get_vttbr() in the same way the arch code > does in cpu_do_switch_mm(). > > I think this saves writing back a cached pre-filtered version every time, or needing > special hooks to know when the value changed. (so we can remove this variable) [...] >> +static void vmid_update_ctxt(void *ctxt) >> { >> + struct kvm_vmid *vmid = ctxt; >> + u64 asid = atomic64_read(&vmid->asid); > >> + vmid->vmid = asid & ((1ULL << kvm_get_vmid_bits()) - 1); > > I don't like having to poke this through the asid-allocator as a kvm-specific hack. Can we > do it in kvm_get_vttbr()? I will have a look. > > >> } > >> @@ -487,48 +467,11 @@ static bool need_new_vmid_gen(struct kvm_vmid *vmid) > > (git made a mess of the diff here... squashed to just the new code:) > >> static void update_vmid(struct kvm_vmid *vmid) >> { > >> + int cpu = get_cpu(); >> >> + asid_check_context(&vmid_info, &vmid->asid, cpu, vmid); >> >> + put_cpu(); > > If we're calling update_vmid() in a pre-emptible context, aren't we already doomed? Yes we are. This made me realize that Linux-RT replaced the preempt_disable() in the caller by migrate_disable(). The latter will prevent the task to move to another CPU but allow preemption. This patch will likely makes things awfully broken for Linux-RT. I will have a look to see if we can call this from preempt notifier. > > Could we use smp_processor_id() instead. > > >> } > > >> @@ -1322,6 +1271,8 @@ static void cpu_init_hyp_mode(void *dummy) >> >> __cpu_init_hyp_mode(pgd_ptr, hyp_stack_ptr, vector_ptr); >> __cpu_init_stage2(); > > >> + kvm_call_hyp(__kvm_flush_cpu_vmid_context); > > I think we only need to do this for VHE systems too. cpu_hyp_reinit() only does the call > to cpu_init_hyp_mode() if !is_kernel_in_hyp_mode(). I guess you mean we need to do this for VHE system. If so, I agree that cpu_init_hyp_mode() is not the best place. I will move it to cpu_hyp_reinit(). > > >> } >> >> static void cpu_hyp_reset(void) >> @@ -1429,6 +1380,17 @@ static inline void hyp_cpu_pm_exit(void) >> >> static int init_common_resources(void) >> { >> + /* >> + * Initialize the ASID allocator telling it to allocate a single >> + * VMID per VM. >> + */ >> + if (asid_allocator_init(&vmid_info, kvm_get_vmid_bits(), 1, >> + vmid_flush_cpu_ctxt, vmid_update_ctxt)) >> + panic("Failed to initialize VMID allocator\n"); > > Couldn't we return an error instead? The asid allocator is needed for user-space, its > pointless to keep running if it fails. The same isn't true here. (and it would make it > easier to debug what went wrong!) Fair point. I will update the next version. Cheers, -- Julien Grall
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