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

Anup Patel
2019-08-22T12:38:47Z

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.

Regards,
Anup

>
>
> Alex