linux-kernel - Re: [PATCH v3 4/6] KVM: MMU: fast invalidate all mmio sptes

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Date:	Thu, 27 Jun 2013 17:50:08 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	avi.kivity@...il.com, mtosatti@...hat.com, pbonzini@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v3 4/6] KVM: MMU: fast invalidate all mmio sptes

On 06/27/2013 05:21 PM, Gleb Natapov wrote:
> On Thu, Jun 27, 2013 at 12:14:24PM +0300, Gleb Natapov wrote:
>> On Thu, Jun 27, 2013 at 12:01:10PM +0300, Gleb Natapov wrote:
>>> On Thu, Jun 27, 2013 at 11:29:00AM +0300, Gleb Natapov wrote:
>>>> On Fri, Jun 07, 2013 at 04:51:26PM +0800, Xiao Guangrong wrote:
>>>>> This patch tries to introduce a very simple and scale way to invalidate
>>>>> all mmio sptes - it need not walk any shadow pages and hold mmu-lock
>>>>>
>>>>> KVM maintains a global mmio valid generation-number which is stored in
>>>>> kvm->memslots.generation and every mmio spte stores the current global
>>>>> generation-number into his available bits when it is created
>>>>>
>>>>> When KVM need zap all mmio sptes, it just simply increase the global
>>>>> generation-number. When guests do mmio access, KVM intercepts a MMIO #PF
>>>>> then it walks the shadow page table and get the mmio spte. If the
>>>>> generation-number on the spte does not equal the global generation-number,
>>>>> it will go to the normal #PF handler to update the mmio spte
>>>>>
>>>>> Since 19 bits are used to store generation-number on mmio spte, we zap all
>>>>> mmio sptes when the number is round
>>>>>
>>>> So this commit makes Fedora 9 32 bit reboot during boot, Fedora 9 64
>>>> fails too, but I haven't checked what happens exactly.
>>>>
>>> Something wrong with gfn calculation during mmio:
>>>
>>> qemu-system-x86-17003 [000]  3962.625103: handle_mmio_page_fault: addr:c00ba6c0 gfn 100000000ba access a92
>>> qemu-system-x86-17003 [000]  3962.774862: handle_mmio_page_fault: addr:ffffb170 gfn 100000fee00 access a92
>>>
>> Hmm, so I wounder why get_mmio_spte_gfn() does not clear gen bits.
>>
> Hmm, something like patch below fixes it. Will test more.
> 
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 1fd2c05..aec9c05 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -260,7 +260,8 @@ static bool is_mmio_spte(u64 spte)
> 
>  static gfn_t get_mmio_spte_gfn(u64 spte)
>  {
> -	return (spte & ~shadow_mmio_mask) >> PAGE_SHIFT;
> +	u64 mask = generation_mmio_spte_mask(MMIO_MAX_GEN) | shadow_mmio_mask;
> +	return (spte & ~mask) >> PAGE_SHIFT;
>  }

Looks nice.

Gleb, thank you very much for investigating the bug and fixing my mistake.
I will be more careful in the further developments.

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