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

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

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.

So, i guess we can do the simple fix first.