Subject: Re: [RFC PATCH v1 2/4] kvmppc: Add support for shared pages in HMM driver

From: Bharata B Rao <[email-masked]>
Date: Mon, 12 Nov 2018 15:37:02 +0530

On Thu, Nov 01, 2018 at 09:45:52PM +1100, Balbir Singh wrote:
> On Mon, Oct 22, 2018 at 10:48:35AM +0530, Bharata B Rao wrote:
> > A secure guest will share some of its pages with hypervisor (Eg. virtio
> > bounce buffers etc). Support shared pages in HMM driver.
> > 
> > Signed-off-by: Bharata B Rao <[email-masked]>
> > ---
> >  arch/powerpc/kvm/book3s_hv_hmm.c | 69 ++++++++++++++++++++++++++++++--
> >  1 file changed, 65 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/powerpc/kvm/book3s_hv_hmm.c b/arch/powerpc/kvm/book3s_hv_hmm.c
> > index a2ee3163a312..09b8e19b7605 100644
> > --- a/arch/powerpc/kvm/book3s_hv_hmm.c
> > +++ b/arch/powerpc/kvm/book3s_hv_hmm.c
> > @@ -50,6 +50,7 @@ struct kvmppc_hmm_page_pvt {
> >  	struct hlist_head *hmm_hash;
> >  	unsigned int lpid;
> >  	unsigned long gpa;
> > +	bool skip_page_out;
> >  };
> >  
> >  struct kvmppc_hmm_migrate_args {
> > @@ -278,6 +279,65 @@ static unsigned long kvmppc_gpa_to_hva(struct kvm *kvm, unsigned long gpa,
> >  	return hva;
> >  }
> >  
> > +/*
> > + * Shares the page with HV, thus making it a normal page.
> > + *
> > + * - If the page is already secure, then provision a new page and share
> > + * - If the page is a normal page, share the existing page
> > + *
> > + * In the former case, uses the HMM fault handler to release the HMM page.
> > + */
> > +static unsigned long
> > +kvmppc_share_page(struct kvm *kvm, unsigned long gpa,
> > +		  unsigned long addr, unsigned long page_shift)
> > +{
> > +
> 
> So this is a special flag passed via the hypercall to say
> this page can be skipped from page_out from secure memory?
> Who has the master copy of the page at this point?
> 
> In which case the question is
> 
> Why did we get a fault on the page which resulted in the
> fault migration ops being called?
> What category of pages are considered shared?

When UV/guest asks for sharing a page, there can be two cases:

- If the page is already secure, then provision a new page and share
- If the page is a normal page, share the existing page

In the former case, we touch the page via get_user_pages() and re-use the
HMM fault handler to release the HMM page. We use skip_page_out to mark
that this page is meant to be released w/o doing a page-out which otherwise
would be done if HV touches a secure page.

When a page is shared, both HV and UV have mappings to the same physical
page that resides in the non-secure memory.

Regards,
Bharata.