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

From: Balbir Singh <[email-masked]>
Date: Thu, 1 Nov 2018 21:45:52 +1100

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?

Balbir