Re: [PATCH v2 (resend) 09/27] x86/pv: Rewrite how building PV dom0 handles domheap mappings

From: Jan Beulich <jbeulich@suse.com>
To: Elias El Yandouzi <eliasely@amazon.com>
Cc: julien@xen.org, pdurrant@amazon.com, dwmw@amazon.com,
	"Hongyan Xia" <hongyxia@amazon.com>,
	"Andrew Cooper" <andrew.cooper3@citrix.com>,
	"Roger Pau Monné" <roger.pau@citrix.com>, "Wei Liu" <wl@xen.org>,
	"Julien Grall" <jgrall@amazon.com>,
	xen-devel@lists.xenproject.org
Subject: Re: [PATCH v2 (resend) 09/27] x86/pv: Rewrite how building PV dom0 handles domheap mappings
Date: Tue, 20 Feb 2024 11:28:32 +0100	[thread overview]
Message-ID: <7031861b-f925-479c-b236-8b1809996ede@suse.com> (raw)
In-Reply-To: <20240116192611.41112-10-eliasely@amazon.com>

On 16.01.2024 20:25, Elias El Yandouzi wrote:
> --- a/xen/arch/x86/pv/dom0_build.c
> +++ b/xen/arch/x86/pv/dom0_build.c
> @@ -382,6 +382,10 @@ int __init dom0_construct_pv(struct domain *d,
>      l3_pgentry_t *l3tab = NULL, *l3start = NULL;
>      l2_pgentry_t *l2tab = NULL, *l2start = NULL;
>      l1_pgentry_t *l1tab = NULL, *l1start = NULL;
> +    mfn_t l4start_mfn = INVALID_MFN;
> +    mfn_t l3start_mfn = INVALID_MFN;
> +    mfn_t l2start_mfn = INVALID_MFN;
> +    mfn_t l1start_mfn = INVALID_MFN;

The reason initializers are needed here is, aiui, the overly large scope
of these variables. For example ...

> @@ -708,22 +712,32 @@ int __init dom0_construct_pv(struct domain *d,
>          v->arch.pv.event_callback_cs    = FLAT_COMPAT_KERNEL_CS;
>      }
>  
> +#define UNMAP_MAP_AND_ADVANCE(mfn_var, virt_var, maddr) \
> +do {                                                    \
> +    unmap_domain_page(virt_var);                        \
> +    mfn_var = maddr_to_mfn(maddr);                      \
> +    maddr += PAGE_SIZE;                                 \
> +    virt_var = map_domain_page(mfn_var);                \
> +} while ( false )
> +
>      if ( !compat )
>      {
>          maddr_to_page(mpt_alloc)->u.inuse.type_info = PGT_l4_page_table;
> -        l4start = l4tab = __va(mpt_alloc); mpt_alloc += PAGE_SIZE;
> +        UNMAP_MAP_AND_ADVANCE(l4start_mfn, l4start, mpt_alloc);
> +        l4tab = l4start;
>          clear_page(l4tab);
> -        init_xen_l4_slots(l4tab, _mfn(virt_to_mfn(l4start)),
> -                          d, INVALID_MFN, true);
> -        v->arch.guest_table = pagetable_from_paddr(__pa(l4start));
> +        init_xen_l4_slots(l4tab, l4start_mfn, d, INVALID_MFN, true);
> +        v->arch.guest_table = pagetable_from_mfn(l4start_mfn);

... looks to be required only here, while ...

>      }
>      else
>      {
>          /* Monitor table already created by switch_compat(). */
> -        l4start = l4tab = __va(pagetable_get_paddr(v->arch.guest_table));
> +        l4start_mfn = pagetable_get_mfn(v->arch.guest_table);
> +        l4start = l4tab = map_domain_page(l4start_mfn);

... in principle the use of the variable could be avoided here. Below
from here there's no further use of it.

> @@ -781,30 +797,34 @@ int __init dom0_construct_pv(struct domain *d,
>  
>      if ( compat )
>      {
> -        l2_pgentry_t *l2t;
> -
>          /* Ensure the first four L3 entries are all populated. */
>          for ( i = 0, l3tab = l3start; i < 4; ++i, ++l3tab )
>          {
>              if ( !l3e_get_intpte(*l3tab) )
>              {
>                  maddr_to_page(mpt_alloc)->u.inuse.type_info = PGT_l2_page_table;
> -                l2tab = __va(mpt_alloc); mpt_alloc += PAGE_SIZE;
> -                clear_page(l2tab);
> -                *l3tab = l3e_from_paddr(__pa(l2tab), L3_PROT);
> +                UNMAP_MAP_AND_ADVANCE(l2start_mfn, l2start, mpt_alloc);
> +                clear_page(l2start);
> +                *l3tab = l3e_from_mfn(l2start_mfn, L3_PROT);
>              }

The updating of l2start is only conditional here, yet ...

>              if ( i == 3 )
>                  l3e_get_page(*l3tab)->u.inuse.type_info |= PGT_pae_xen_l2;
>          }
>  
> -        l2t = map_l2t_from_l3e(l3start[3]);
> -        init_xen_pae_l2_slots(l2t, d);
> -        unmap_domain_page(l2t);
> +        init_xen_pae_l2_slots(l2start, d);

... here you assume it points at the page referenced by the 3rd L3 entry.
Question is why the original code is being replaced here in the first
place: It was already suitably mapping the page in question.

Jan