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

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, Sep 22, 2009 at 09:59:04AM +0300, Avi Kivity wrote:
> 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?

No:

kvm_vm_ioctl_set_nr_mmu_pages -> kvm_mmu_change_mmu_pages

And even if the caller did, recursive "locking" is tolerated.

>> 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?  

kvm_arch_vcpu_ioctl_set_sregs -> kvm_x86_ops->set_cr0.

> 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.

Hum, possible, but i'd rather leave it for later.