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

[Anup Patel]

On Thu, Aug 22, 2019 at 5:58 PM Alexander Graf <graf@xxxxxxxxxx> 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@xxxxxxx>
> > Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Reviewed-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > ---
> >   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.

Regards,
Anup

>
>
> Alex