linux-kernel - Re: [PATCH 2/2] KVM: x86: map/unmap private slots in __x86_set_memory

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Message-ID: <561D3126.8060402@redhat.com>
Date:	Tue, 13 Oct 2015 18:28:22 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Radim Krčmář <rkrcmar@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	aderumier@...so.com, stable@...r.kernel.org
Subject: Re: [PATCH 2/2] KVM: x86: map/unmap private slots in
 __x86_set_memory_region



On 13/10/2015 17:39, Radim Krčmář wrote:
> 2015-10-12 14:09+0200, Paolo Bonzini:
>> Otherwise, two copies (one of them never used and thus bogus) are
>> allocated for the regular and SMM address spaces.  This breaks
>> SMM with EPT but without unrestricted guest support, because the
>> SMM copy of the identity page map is all zeros.
> 
> (Have you found out why EPT+unrestricted didn't use the alternative SMM
>  mapping as well?)

Yes, that I already knew; EPT+unrestricted uses CR0.PG=0 directly so
it doesn't use the identity page at all.  (CR0.PG=0 w/o unrestricted
instead runs with CR0.PG=1.  CR3 load and store exits are enabled,
and the guest CR3 always points to the identity page map while the
guest runs).

>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> @@ -7717,23 +7717,53 @@ void kvm_arch_sync_events(struct kvm *kvm)
>>  int __x86_set_memory_region(struct kvm *kvm, int id, gpa_t gpa, u32 size)
>>  {
>>  	int i, r;
>> +	u64 hva;
>> +	struct kvm_memslots *slots = kvm_memslots(kvm);
>> +	struct kvm_memory_slot *slot, old;
> | [...]
>> +	slot = &slots->memslots[slots->id_to_index[id]];
> 
> This seems better written as
> 
>    	slot = id_to_memslot(slots, id);

Gah, I could not recall the right API!  I'll fix it.

> (Made me remember that I want to refactor the memslot API ...)
> 
> | [...]
>> +	} else {
>> +		if (!slot->npages)
>> +			return 0;
>> +
>> +		hva = 0;
>> +	}
>> +
>> +	old = *slot;
> 
> (Assignment could be in the 'else' == !size branch, GCC would have fun.)

It would have fun _and_ warn, which is why it's not in the else branch. :)

Paolo
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