On Feb 15, 2013, at 1:41 PM, Gleb Natapov wrote:
> On Fri, Feb 15, 2013 at 01:19:29PM -0500, Sanjay Lal wrote:
>>
>> On Feb 6, 2013, at 7:08 AM, Gleb Natapov wrote:
>>
>>>>
>>>> +static void kvm_mips_map_page(struct kvm *kvm, gfn_t gfn)
>>>> +{
>>>> + pfn_t pfn;
>>>> +
>>>> + if (kvm->arch.guest_pmap[gfn] != KVM_INVALID_PAGE)
>>>> + return;
>>>> +
>>>> + pfn =kvm_mips_gfn_to_pfn(kvm, gfn);
>>> This call should be in srcu read section since it access memory slots which
>>> are srcu protected. You should test with RCU debug enabled.
>>
>> kvm_mips_gfn_to_pfn just maps to gfn_to_pfn. I don't see an instance where gfn_to_pfn is in a scru read section?
>>
> Where are you looking? On x86 if vcpu is not in a guest mode it is in
> srcu read section. The lock is taken immediately after exit and released
> before entry. This is because x86 access memory slots a lot. Other
> arches, that do not access memory slots as much, take the lock around
> each individual access. As far as I see this is the only place in MIPS
> kvm where this is needed.
Ah I see what you mean. I'll wrap the access in a scru read section.
>
>>>
>>>>
>>>> +
>>>> +uint32_t kvm_get_inst(uint32_t *opc, struct kvm_vcpu *vcpu)
>>>> +{
>>>> + uint32_t inst;
>>>> + struct mips_coproc *cop0 __unused = vcpu->arch.cop0;
>>>> + int index;
>>>> + ulong paddr, flags;
>>>> +
>>>> + if (KVM_GUEST_KSEGX((ulong) opc) < KVM_GUEST_KSEG0 ||
>>>> + KVM_GUEST_KSEGX((ulong) opc) == KVM_GUEST_KSEG23) {
>>>> + local_irq_save(flags);
>>>> + index = kvm_mips_host_tlb_lookup(vcpu, (ulong) opc);
>>>> + if (index >= 0) {
>>>> + inst = *(opc);
>>> Here and in some more places below you access __user memory. Shouldn't you
>>> use get_user() to access it? What prevents the kernel crash by access fault here
>>> if userspace remaps the memory to be non-readable? Hmm, may be it uses
>>> guest translation here so it cannot happen, but still, sparse will not
>>> be happy and kvm_mips_translate_guest_kseg0_to_hpa() case below uses
>>> host translation anyway.
>>>
>> Actually, I don't need the __user declaration in most cases, since KVM/MIPS handles mapping the page (if needed) and does not rely on the usual kernel mechanisms.
> Yes I see that you check/populate tlb for non KVM_GUEST_KSEG0 access,
> for kvm_mips_translate_guest_kseg0_to_hpa() you do not, but now I notice
> that you are not using the address directly but uses CKSEG0ADDR() on it,
> which, as far as I can tell, gives you kernel mapping for the page,
> correct? Why this is better than using get_user()? To save tlb entries?
>
Couple of reasons, KVM/MIPS uses its own TLB handlers while the guest is running, so get_user is not an option, and (2) it does save on TLBs when accessing the guest memory via KSEG0.
