From: Jan Beulich <jbeulich@suse.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>, Tim Deegan <tim@xen.org>
Cc: Xen-devel <xen-devel@lists.xenproject.org>,
"Wei Liu" <wl@xen.org>, "Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [PATCH] x86/traps: Rework #PF[Rsvd] bit handling
Date: Tue, 19 May 2020 18:09:55 +0200 [thread overview]
Message-ID: <4a5c33c0-9245-126b-123e-3980a9135190@suse.com> (raw)
In-Reply-To: <1750cbe5-ef48-6dc7-e372-cbc0a8cbc9cc@citrix.com>
On 19.05.2020 17:33, Andrew Cooper wrote:
> On 19/05/2020 15:48, Jan Beulich wrote:
>> On 19.05.2020 16:11, Andrew Cooper wrote:
>>> Given that shadow frames are limited to 44 bits anyway (and not yet
>>> levelled safely in the migration stream), my suggestion for fixing this
>>> was just to use one extra nibble for the extra 4 bits and call it done.
>> Would you remind(?) me of where this 44-bit restriction is coming
>> from?
>
> From paging_max_paddr_bits(),
>
> /* Shadowed superpages store GFNs in 32-bit page_info fields. */
Ah, that's an abuse of the backlink field. After some looking
around I first thought the up field could be used to store the
GFN instead, as it's supposedly used for single-page shadows
only. Then however I found
static inline int sh_type_has_up_pointer(struct domain *d, unsigned int t)
{
/* Multi-page shadows don't have up-pointers */
if ( t == SH_type_l1_32_shadow
|| t == SH_type_fl1_32_shadow
|| t == SH_type_l2_32_shadow )
return 0;
/* Pinnable shadows don't have up-pointers either */
return !sh_type_is_pinnable(d, t);
}
It's unclear to me in which way SH_type_l1_32_shadow and
SH_type_l2_32_shadow are "multi-page" shadows; I'd rather have
expected all three SH_type_fl1_*_shadow to be. Tim?
In any event there would be 12 bits to reclaim from the up
pointer - it being a physical address, there'll not be more
than 52 significant bits.
Jan
next prev parent reply other threads:[~2020-05-19 16:10 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-18 15:38 [PATCH] x86/traps: Rework #PF[Rsvd] bit handling Andrew Cooper
2020-05-18 15:40 ` Andrew Cooper
2020-05-19 8:14 ` Jan Beulich
2020-05-19 14:29 ` Andrew Cooper
2020-05-19 14:55 ` Jan Beulich
2020-05-19 15:59 ` Andrew Cooper
2020-05-19 8:34 ` Jan Beulich
2020-05-19 14:11 ` Andrew Cooper
2020-05-19 14:48 ` Jan Beulich
2020-05-19 15:33 ` Andrew Cooper
2020-05-19 16:09 ` Jan Beulich [this message]
2020-05-19 18:00 ` Andrew Cooper
2020-05-20 7:48 ` Jan Beulich
2020-05-20 15:48 ` Andrew Cooper
2020-05-20 7:10 ` Tim Deegan
2020-05-21 15:43 ` [PATCH v2] " Andrew Cooper
2020-05-22 13:51 ` Jan Beulich
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