Re: [PATCH 0/2] kvm: IOMMU read-only mapping support

[Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>]

On 01/29/2013 02:50 PM, Gleb Natapov wrote:
> On Tue, Jan 29, 2013 at 11:06:43AM +0800, Xiao Guangrong wrote:
>> On 01/28/2013 06:59 PM, Gleb Natapov wrote:
>>> On Fri, Jan 25, 2013 at 11:28:40AM +0800, Xiao Guangrong wrote:
>>>> On 01/25/2013 09:17 AM, Takuya Yoshikawa wrote:
>>>>> On Thu, 24 Jan 2013 15:03:57 -0700
>>>>> Alex Williamson <alex.williamson@redhat.com> wrote:
>>>>>
>>>>>> A couple patches to make KVM IOMMU support honor read-only mappings.
>>>>>> This causes an un-map, re-map when the read-only flag changes and
>>>>>> makes use of it when setting IOMMU attributes.  Thanks,
>>>>>
>>>>> Looks good to me.
>>>>>
>>>>> I think I can naturally update my patch after this gets merged.
>>>>>
>>>>
>>>> Please wait.
>>>>
>>>> The commit c972f3b1 changed the write-protect behaviour - it does
>>>> wirte-protection only when dirty flag is set.
>>>> [ I did not see this commit when we discussed the problem before. ]
>>>>
>>>> Further more, i notice that write-protect is not enough, when do sync
>>>> shadow page:
>>>>
>>>> FNAME(sync_page):
>>>>
>>>> 		host_writable = sp->spt[i] & SPTE_HOST_WRITEABLE;
>>>>
>>>> 		set_spte(vcpu, &sp->spt[i], pte_access,
>>>> 			 PT_PAGE_TABLE_LEVEL, gfn,
>>>> 			 spte_to_pfn(sp->spt[i]), true, false,
>>>> 			 host_writable);
>>>>
>>>> It sets spte based on the old value that means the readonly flag check
>>>> is missed. We need to call kvm_arch_flush_shadow_all under this case.
>>> Why not just disallow changing memory region KVM_MEM_READONLY flag
>>> without deleting the region?
>>
>> It will introduce some restriction when VM-sharing-mem is being implemented,
>> but we need to do some optimization for it, at least, properly write-protect
>> readonly pages (fix sync_page()) instead of zap_all_page.
>>
> What is VM-sharing-mem?

Sharing memory between different guests.

> 
>> So, i guess we can do the simple fix first.
>>
> By simple fix you mean calling kvm_arch_flush_shadow_all() on READONLY
> flag change?

Simply disallow READONLY flag changing.