Re: [PATCH v5 13/20] RISC-V: KVM: Implement stage2 page table programming

[Anup Patel <anup@brainfault.org>]

On Thu, Aug 22, 2019 at 7:39 PM Alexander Graf <graf@amazon.com> wrote:
>
>
>
> On 22.08.19 15:58, Anup Patel wrote:
> > On Thu, Aug 22, 2019 at 6:57 PM Alexander Graf <graf@amazon.com> wrote:
> >>
> >>
> >>
> >> On 22.08.19 14:38, Anup Patel wrote:
> >>> On Thu, Aug 22, 2019 at 5:58 PM Alexander Graf <graf@amazon.com> wrote:
> >>>>
> >>>> On 22.08.19 10:45, Anup Patel wrote:
> >>>>> This patch implements all required functions for programming
> >>>>> the stage2 page table for each Guest/VM.
> >>>>>
> >>>>> At high-level, the flow of stage2 related functions is similar
> >>>>> from KVM ARM/ARM64 implementation but the stage2 page table
> >>>>> format is quite different for KVM RISC-V.
> >>>>>
> >>>>> Signed-off-by: Anup Patel <anup.patel@wdc.com>
> >>>>> Acked-by: Paolo Bonzini <pbonzini@redhat.com>
> >>>>> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
> >>>>> ---
> >>>>>     arch/riscv/include/asm/kvm_host.h     |  10 +
> >>>>>     arch/riscv/include/asm/pgtable-bits.h |   1 +
> >>>>>     arch/riscv/kvm/mmu.c                  | 637 +++++++++++++++++++++++++-
> >>>>>     3 files changed, 638 insertions(+), 10 deletions(-)
> >>>>>
> >>>>> diff --git a/arch/riscv/include/asm/kvm_host.h b/arch/riscv/include/asm/kvm_host.h
> >>>>> index 3b09158f80f2..a37775c92586 100644
> >>>>> --- a/arch/riscv/include/asm/kvm_host.h
> >>>>> +++ b/arch/riscv/include/asm/kvm_host.h
> >>>>> @@ -72,6 +72,13 @@ struct kvm_mmio_decode {
> >>>>>         int shift;
> >>>>>     };
> >>>>>
> >>>>> +#define KVM_MMU_PAGE_CACHE_NR_OBJS   32
> >>>>> +
> >>>>> +struct kvm_mmu_page_cache {
> >>>>> +     int nobjs;
> >>>>> +     void *objects[KVM_MMU_PAGE_CACHE_NR_OBJS];
> >>>>> +};
> >>>>> +
> >>>>>     struct kvm_cpu_context {
> >>>>>         unsigned long zero;
> >>>>>         unsigned long ra;
> >>>>> @@ -163,6 +170,9 @@ struct kvm_vcpu_arch {
> >>>>>         /* MMIO instruction details */
> >>>>>         struct kvm_mmio_decode mmio_decode;
> >>>>>
> >>>>> +     /* Cache pages needed to program page tables with spinlock held */
> >>>>> +     struct kvm_mmu_page_cache mmu_page_cache;
> >>>>> +
> >>>>>         /* VCPU power-off state */
> >>>>>         bool power_off;
> >>>>>
> >>>>> diff --git a/arch/riscv/include/asm/pgtable-bits.h b/arch/riscv/include/asm/pgtable-bits.h
> >>>>> index bbaeb5d35842..be49d62fcc2b 100644
> >>>>> --- a/arch/riscv/include/asm/pgtable-bits.h
> >>>>> +++ b/arch/riscv/include/asm/pgtable-bits.h
> >>>>> @@ -26,6 +26,7 @@
> >>>>>
> >>>>>     #define _PAGE_SPECIAL   _PAGE_SOFT
> >>>>>     #define _PAGE_TABLE     _PAGE_PRESENT
> >>>>> +#define _PAGE_LEAF      (_PAGE_READ | _PAGE_WRITE | _PAGE_EXEC)
> >>>>>
> >>>>>     /*
> >>>>>      * _PAGE_PROT_NONE is set on not-present pages (and ignored by the hardware) to
> >>>>> diff --git a/arch/riscv/kvm/mmu.c b/arch/riscv/kvm/mmu.c
> >>>>> index 2b965f9aac07..9e95ab6769f6 100644
> >>>>> --- a/arch/riscv/kvm/mmu.c
> >>>>> +++ b/arch/riscv/kvm/mmu.c
> >>>>> @@ -18,6 +18,432 @@
> >>>>>     #include <asm/page.h>
> >>>>>     #include <asm/pgtable.h>
> >>>>>
> >>>>> +#ifdef CONFIG_64BIT
> >>>>> +#define stage2_have_pmd              true
> >>>>> +#define stage2_gpa_size              ((phys_addr_t)(1ULL << 39))
> >>>>> +#define stage2_cache_min_pages       2
> >>>>> +#else
> >>>>> +#define pmd_index(x)         0
> >>>>> +#define pfn_pmd(x, y)                ({ pmd_t __x = { 0 }; __x; })
> >>>>> +#define stage2_have_pmd              false
> >>>>> +#define stage2_gpa_size              ((phys_addr_t)(1ULL << 32))
> >>>>> +#define stage2_cache_min_pages       1
> >>>>> +#endif
> >>>>> +
> >>>>> +static int stage2_cache_topup(struct kvm_mmu_page_cache *pcache,
> >>>>> +                           int min, int max)
> >>>>> +{
> >>>>> +     void *page;
> >>>>> +
> >>>>> +     BUG_ON(max > KVM_MMU_PAGE_CACHE_NR_OBJS);
> >>>>> +     if (pcache->nobjs >= min)
> >>>>> +             return 0;
> >>>>> +     while (pcache->nobjs < max) {
> >>>>> +             page = (void *)__get_free_page(GFP_KERNEL | __GFP_ZERO);
> >>>>> +             if (!page)
> >>>>> +                     return -ENOMEM;
> >>>>> +             pcache->objects[pcache->nobjs++] = page;
> >>>>> +     }
> >>>>> +
> >>>>> +     return 0;
> >>>>> +}
> >>>>> +
> >>>>> +static void stage2_cache_flush(struct kvm_mmu_page_cache *pcache)
> >>>>> +{
> >>>>> +     while (pcache && pcache->nobjs)
> >>>>> +             free_page((unsigned long)pcache->objects[--pcache->nobjs]);
> >>>>> +}
> >>>>> +
> >>>>> +static void *stage2_cache_alloc(struct kvm_mmu_page_cache *pcache)
> >>>>> +{
> >>>>> +     void *p;
> >>>>> +
> >>>>> +     if (!pcache)
> >>>>> +             return NULL;
> >>>>> +
> >>>>> +     BUG_ON(!pcache->nobjs);
> >>>>> +     p = pcache->objects[--pcache->nobjs];
> >>>>> +
> >>>>> +     return p;
> >>>>> +}
> >>>>> +
> >>>>> +struct local_guest_tlb_info {
> >>>>> +     struct kvm_vmid *vmid;
> >>>>> +     gpa_t addr;
> >>>>> +};
> >>>>> +
> >>>>> +static void local_guest_tlb_flush_vmid_gpa(void *info)
> >>>>> +{
> >>>>> +     struct local_guest_tlb_info *infop = info;
> >>>>> +
> >>>>> +     __kvm_riscv_hfence_gvma_vmid_gpa(READ_ONCE(infop->vmid->vmid_version),
> >>>>> +                                      infop->addr);
> >>>>> +}
> >>>>> +
> >>>>> +static void stage2_remote_tlb_flush(struct kvm *kvm, gpa_t addr)
> >>>>> +{
> >>>>> +     struct local_guest_tlb_info info;
> >>>>> +     struct kvm_vmid *vmid = &kvm->arch.vmid;
> >>>>> +
> >>>>> +     /* TODO: This should be SBI call */
> >>>>> +     info.vmid = vmid;
> >>>>> +     info.addr = addr;
> >>>>> +     preempt_disable();
> >>>>> +     smp_call_function_many(cpu_all_mask, local_guest_tlb_flush_vmid_gpa,
> >>>>> +                            &info, true);
> >>>>
> >>>> This is all nice and dandy on the toy 4 core systems we have today, but
> >>>> it will become a bottleneck further down the road.
> >>>>
> >>>> How many VMIDs do you have? Could you just allocate a new one every time
> >>>> you switch host CPUs? Then you know exactly which CPUs to flush by
> >>>> looking at all your vcpu structs and a local field that tells you which
> >>>> pCPU they're on at this moment.
> >>>>
> >>>> Either way, it's nothing that should block inclusion. For today, we're fine.
> >>>
> >>> We are not happy about this either.
> >>>
> >>> Other two options, we have are:
> >>> 1. Have SBI calls for remote HFENCEs
> >>> 2. Propose RISC-V ISA extension for remote FENCEs
> >>>
> >>> Option1 is mostly extending SBI spec and implementing it in runtime
> >>> firmware.
> >>>
> >>> Option2 is ideal solution but requires consensus among wider audience
> >>> in RISC-V foundation.
> >>>
> >>> At this point, we are fine with a simple solution.
> >>
> >> It's fine to explicitly IPI other CPUs to flush their TLBs. What is not
> >> fine is to IPI *all* CPUs to flush their TLBs.
> >
> > Ahh, this should have been cpu_online_mask instead of cpu_all_mask
> >
> > I will update this in next revision.
>
> What I was trying to say is that you only want to flush currently
> running other vcpus and add a hint for all the others saying "please
> flush the next time you come up".
>
> I think we had a mechanism for that somewhere in the EVENT magic.
>
> But as I said, this is a performance optimization - that's something I'm
> happy to delay. Security and user space ABI are the bits I'm worried
> about at this stage.

I had thought about this previously. This will be certainly a good
optimization. Let me add a TODO comment here so that we don't
forget it.

Regards,
Anup

>
>
> Alex