linux-kernel - Re: [PATCH v4 06/14] KVM: s390: pv: properly handle page flags for protected guests

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Date:   Mon, 6 Sep 2021 17:56:18 +0200
From:   Claudio Imbrenda <imbrenda@...ux.ibm.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>
Cc:     kvm@...r.kernel.org, cohuck@...hat.com, frankja@...ux.ibm.com,
        thuth@...hat.com, pasic@...ux.ibm.com, david@...hat.com,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ulrich.Weigand@...ibm.com
Subject: Re: [PATCH v4 06/14] KVM: s390: pv: properly handle page flags for
 protected guests

On Mon, 6 Sep 2021 17:46:40 +0200
Christian Borntraeger <borntraeger@...ibm.com> wrote:

> On 18.08.21 15:26, Claudio Imbrenda wrote:
> > Introduce variants of the convert and destroy page functions that also
> > clear the PG_arch_1 bit used to mark them as secure pages.
> > 
> > These new functions can only be called on pages for which a reference
> > is already being held.
> > 
> > Signed-off-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>
> > Acked-by: Janosch Frank <frankja@...ux.ibm.com>  
> 
> Can you refresh my mind? We do have over-indication of PG_arch_1 and this
> might result in spending some unneeded cycles but in the end this will be
> correct. Right?
> And this patch will fix some unnecessary places that add overindication.

correct, PG_arch_1 will still overindicate, but with this patch it will
happen less.

And PG_arch_1 overindication is perfectly fine from a correctness point
of view.

> > ---
> >   arch/s390/include/asm/pgtable.h |  9 ++++++---
> >   arch/s390/include/asm/uv.h      | 10 ++++++++--
> >   arch/s390/kernel/uv.c           | 34 ++++++++++++++++++++++++++++++++-
> >   arch/s390/mm/gmap.c             |  4 +++-
> >   4 files changed, 50 insertions(+), 7 deletions(-)
> > 
> > diff --git a/arch/s390/include/asm/pgtable.h b/arch/s390/include/asm/pgtable.h
> > index dcac7b2df72c..0f1af2232ebe 100644
> > --- a/arch/s390/include/asm/pgtable.h
> > +++ b/arch/s390/include/asm/pgtable.h
> > @@ -1074,8 +1074,9 @@ static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
> >   	pte_t res;
> >   
> >   	res = ptep_xchg_lazy(mm, addr, ptep, __pte(_PAGE_INVALID));
> > +	/* At this point the reference through the mapping is still present */
> >   	if (mm_is_protected(mm) && pte_present(res))
> > -		uv_convert_from_secure(pte_val(res) & PAGE_MASK);
> > +		uv_convert_owned_from_secure(pte_val(res) & PAGE_MASK);
> >   	return res;
> >   }
> >   
> > @@ -1091,8 +1092,9 @@ static inline pte_t ptep_clear_flush(struct vm_area_struct *vma,
> >   	pte_t res;
> >   
> >   	res = ptep_xchg_direct(vma->vm_mm, addr, ptep, __pte(_PAGE_INVALID));
> > +	/* At this point the reference through the mapping is still present */
> >   	if (mm_is_protected(vma->vm_mm) && pte_present(res))
> > -		uv_convert_from_secure(pte_val(res) & PAGE_MASK);
> > +		uv_convert_owned_from_secure(pte_val(res) & PAGE_MASK);
> >   	return res;
> >   }
> >   
> > @@ -1116,8 +1118,9 @@ static inline pte_t ptep_get_and_clear_full(struct mm_struct *mm,
> >   	} else {
> >   		res = ptep_xchg_lazy(mm, addr, ptep, __pte(_PAGE_INVALID));
> >   	}
> > +	/* At this point the reference through the mapping is still present */
> >   	if (mm_is_protected(mm) && pte_present(res))
> > -		uv_convert_from_secure(pte_val(res) & PAGE_MASK);
> > +		uv_convert_owned_from_secure(pte_val(res) & PAGE_MASK);
> >   	return res;
> >   }
> >   
> > diff --git a/arch/s390/include/asm/uv.h b/arch/s390/include/asm/uv.h
> > index b35add51b967..3236293d5a31 100644
> > --- a/arch/s390/include/asm/uv.h
> > +++ b/arch/s390/include/asm/uv.h
> > @@ -356,8 +356,9 @@ static inline int is_prot_virt_host(void)
> >   }
> >   
> >   int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb);
> > -int uv_destroy_page(unsigned long paddr);
> > +int uv_destroy_owned_page(unsigned long paddr);
> >   int uv_convert_from_secure(unsigned long paddr);
> > +int uv_convert_owned_from_secure(unsigned long paddr);
> >   int gmap_convert_to_secure(struct gmap *gmap, unsigned long gaddr);
> >   
> >   void setup_uv(void);
> > @@ -367,7 +368,7 @@ void adjust_to_uv_max(unsigned long *vmax);
> >   static inline void setup_uv(void) {}
> >   static inline void adjust_to_uv_max(unsigned long *vmax) {}
> >   
> > -static inline int uv_destroy_page(unsigned long paddr)
> > +static inline int uv_destroy_owned_page(unsigned long paddr)
> >   {
> >   	return 0;
> >   }
> > @@ -376,6 +377,11 @@ static inline int uv_convert_from_secure(unsigned long paddr)
> >   {
> >   	return 0;
> >   }
> > +
> > +static inline int uv_convert_owned_from_secure(unsigned long paddr)
> > +{
> > +	return 0;
> > +}
> >   #endif
> >   
> >   #if defined(CONFIG_PROTECTED_VIRTUALIZATION_GUEST) || IS_ENABLED(CONFIG_KVM)
> > diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
> > index 68a8fbafcb9c..05f8bf61d20a 100644
> > --- a/arch/s390/kernel/uv.c
> > +++ b/arch/s390/kernel/uv.c
> > @@ -115,7 +115,7 @@ static int uv_pin_shared(unsigned long paddr)
> >    *
> >    * @paddr: Absolute host address of page to be destroyed
> >    */
> > -int uv_destroy_page(unsigned long paddr)
> > +static int uv_destroy_page(unsigned long paddr)
> >   {
> >   	struct uv_cb_cfs uvcb = {
> >   		.header.cmd = UVC_CMD_DESTR_SEC_STOR,
> > @@ -135,6 +135,22 @@ int uv_destroy_page(unsigned long paddr)
> >   	return 0;
> >   }
> >   
> > +/*
> > + * The caller must already hold a reference to the page
> > + */
> > +int uv_destroy_owned_page(unsigned long paddr)
> > +{
> > +	struct page *page = phys_to_page(paddr);
> > +	int rc;
> > +
> > +	get_page(page);
> > +	rc = uv_destroy_page(paddr);
> > +	if (!rc)
> > +		clear_bit(PG_arch_1, &page->flags);
> > +	put_page(page);
> > +	return rc;
> > +}
> > +
> >   /*
> >    * Requests the Ultravisor to encrypt a guest page and make it
> >    * accessible to the host for paging (export).
> > @@ -154,6 +170,22 @@ int uv_convert_from_secure(unsigned long paddr)
> >   	return 0;
> >   }
> >   
> > +/*
> > + * The caller must already hold a reference to the page
> > + */
> > +int uv_convert_owned_from_secure(unsigned long paddr)
> > +{
> > +	struct page *page = phys_to_page(paddr);
> > +	int rc;
> > +
> > +	get_page(page);
> > +	rc = uv_convert_from_secure(paddr);
> > +	if (!rc)
> > +		clear_bit(PG_arch_1, &page->flags);
> > +	put_page(page);
> > +	return rc;
> > +}
> > +
> >   /*
> >    * Calculate the expected ref_count for a page that would otherwise have no
> >    * further pins. This was cribbed from similar functions in other places in
> > diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
> > index 5a138f6220c4..38b792ab57f7 100644
> > --- a/arch/s390/mm/gmap.c
> > +++ b/arch/s390/mm/gmap.c
> > @@ -2678,8 +2678,10 @@ static int __s390_reset_acc(pte_t *ptep, unsigned long addr,
> >   {
> >   	pte_t pte = READ_ONCE(*ptep);
> >   
> > +	/* There is a reference through the mapping */
> >   	if (pte_present(pte))
> > -		WARN_ON_ONCE(uv_destroy_page(pte_val(pte) & PAGE_MASK));
> > +		WARN_ON_ONCE(uv_destroy_owned_page(pte_val(pte) & PAGE_MASK));
> > +
> >   	return 0;
> >   }
> >   
> >