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

[Alexander Graf <graf@amazon.com>]

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.


Alex