by Julien Grall

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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