Re: [patch 07/10] KVM: introduce kvm->srcu and convert kvm_set_memory_region to SRCU update

[Avi Kivity <avi@redhat.com>]

On 09/22/2009 02:37 AM, Marcelo Tosatti wrote:
> Use two steps for memslot deletion: mark the slot invalid (which stops
> instantiation of new shadow pages for that slot, but allows destruction),
> then instantiate the new empty slot.
>
> Also simplifies kvm_handle_hva locking.
>
>   unsigned int kvm_mmu_calculate_mmu_pages(struct kvm *kvm)
>   {
> -	int i;
> +	int i, idx;
>   	unsigned int nr_mmu_pages;
>   	unsigned int  nr_pages = 0;
> +	struct kvm_memslots *slots;
>
> -	for (i = 0; i<  kvm->memslots->nmemslots; i++)
> -		nr_pages += kvm->memslots->memslots[i].npages;
> +	idx = srcu_read_lock(&kvm->srcu);
>    

Doesn't the caller hold the srcu_read_lock() here?

>
> Index: kvm-slotslock/arch/x86/kvm/vmx.c
> ===================================================================
> --- kvm-slotslock.orig/arch/x86/kvm/vmx.c
> +++ kvm-slotslock/arch/x86/kvm/vmx.c
> @@ -24,6 +24,7 @@
>   #include<linux/mm.h>
>   #include<linux/highmem.h>
>   #include<linux/sched.h>
> +#include<linux/srcu.h>
>   #include<linux/moduleparam.h>
>   #include<linux/ftrace_event.h>
>   #include "kvm_cache_regs.h"
> @@ -1465,10 +1466,18 @@ static void enter_pmode(struct kvm_vcpu
>   static gva_t rmode_tss_base(struct kvm *kvm)
>   {
>   	if (!kvm->arch.tss_addr) {
> -		gfn_t base_gfn = kvm->memslots->memslots[0].base_gfn +
> -				 kvm->memslots->memslots[0].npages - 3;
> +		struct kvm_memslots *slots;
> +		gfn_t base_gfn;
> +		int idx;
> +
> +		idx = srcu_read_lock(&kvm->srcu);
> +		slots = rcu_dereference(kvm->memslots);
> + 		base_gfn = slots->memslots[0].base_gfn +
> +				 slots->memslots[0].npages - 3;
> +		srcu_read_unlock(&kvm->srcu, idx);
>   		return base_gfn<<  PAGE_SHIFT;
>   	}
> +
>    

And here?  Maybe we should take the srcu_lock in vcpu_load/put and only 
drop in when going into vcpu context or explicitly sleeping, just to 
simplify things.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.