Re: [PATCH 5/6] kvm, x86: use ro page and don't copy shared page

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, Jul 16, 2010 at 10:19:36AM +0300, Gleb Natapov wrote:
> On Fri, Jul 16, 2010 at 10:13:07AM +0800, Lai Jiangshan wrote:
> > When page fault, we always call get_user_pages(write=1).
> > 
> > Actually, we don't need to do this when it is not write fault.
> > get_user_pages(write=1) will cause shared page(ksm) copied.
> > If this page is not modified in future, this copying and the copied page
> > are just wasted. Ksm may scan and merge them and may cause thrash.
> > 
> But is page is written into afterwords we will get another page fault.
> 
> > In this patch, if the page is RO for host VMM and it not write fault for guest,
> > we will use RO page, otherwise we use a writable page.
> > 
> Currently pages allocated for guest memory are required to be RW, so after your series
> behaviour will remain exactly the same as before.

Except KSM pages.

> > Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
> > ---
> > diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> > index 8ba9b0d..6382140 100644
> > --- a/arch/x86/kvm/mmu.c
> > +++ b/arch/x86/kvm/mmu.c
> > @@ -1832,6 +1832,45 @@ static void kvm_unsync_pages(struct kvm_vcpu *vcpu,  gfn_t gfn)
> >  	}
> >  }
> >  
> > +/* get a current mapped page fast, and test whether the page is writable. */
> > +static struct page *get_user_page_and_protection(unsigned long addr,
> > +	int *writable)
> > +{
> > +	struct page *page[1];
> > +
> > +	if (__get_user_pages_fast(addr, 1, 1, page) == 1) {
> > +		*writable = 1;
> > +		return page[0];
> > +	}
> > +	if (__get_user_pages_fast(addr, 1, 0, page) == 1) {
> > +		*writable = 0;
> > +		return page[0];
> > +	}
> > +	return NULL;
> > +}
> > +
> > +static pfn_t kvm_get_pfn_for_page_fault(struct kvm *kvm, gfn_t gfn,
> > +		int write_fault, int *host_writable)
> > +{
> > +	unsigned long addr;
> > +	struct page *page;
> > +
> > +	if (!write_fault) {
> > +		addr = gfn_to_hva(kvm, gfn);
> > +		if (kvm_is_error_hva(addr)) {
> > +			get_page(bad_page);
> > +			return page_to_pfn(bad_page);
> > +		}
> > +
> > +		page = get_user_page_and_protection(addr, host_writable);
> > +		if (page)
> > +			return page_to_pfn(page);
> > +	}
> > +
> > +	*host_writable = 1;
> > +	return kvm_get_pfn_for_gfn(kvm, gfn);
> > +}
> > +
> kvm_get_pfn_for_gfn() returns fault_page if page is mapped RO, so caller
> of kvm_get_pfn_for_page_fault() and kvm_get_pfn_for_gfn() will get
> different results when called on the same page. Not good.
> kvm_get_pfn_for_page_fault() logic should be folded into
> kvm_get_pfn_for_gfn().

Agreed. Please keep gfn_to_pfn related code in virt/kvm/kvm_main.c.