xen-devel.lists.xenproject.org archive mirror
 help / color / mirror / Atom feed
From: Jan Beulich <jbeulich@suse.com>
To: Tim Deegan <tim@xen.org>
Cc: "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	"Andrew Cooper" <andrew.cooper3@citrix.com>,
	"Wei Liu" <wl@xen.org>, "Roger Pau Monné" <roger.pau@citrix.com>,
	"George Dunlap" <george.dunlap@citrix.com>,
	"Kevin Tian" <kevin.tian@intel.com>,
	"Jun Nakajima" <jun.nakajima@intel.com>
Subject: Re: [PATCH v4] VMX: use a single, global APIC access page
Date: Mon, 19 Apr 2021 13:25:26 +0200	[thread overview]
Message-ID: <d698a29e-fb4e-a650-9e48-ad209e64c800@suse.com> (raw)
In-Reply-To: <YHs2AJ6pqrAw7uew@deinos.phlegethon.org>

On 17.04.2021 21:24, Tim Deegan wrote:
> At 12:40 +0200 on 12 Apr (1618231248), Jan Beulich wrote:
>> The address of this page is used by the CPU only to recognize when to
>> access the virtual APIC page instead. No accesses would ever go to this
>> page. It only needs to be present in the (CPU) page tables so that
>> address translation will produce its address as result for respective
>> accesses.
>>
>> By making this page global, we also eliminate the need to refcount it,
>> or to assign it to any domain in the first place.
> 
> What is the aim here?  To save 4k per domain?  It seems to come out
> about even for adding and removing code. 

True, but still it looks wrong to me to use a page per guest when one
her host suffices. Think about many tiny, short-lived VMs (as in
Tamas'es VM forking).

>> --- a/xen/arch/x86/mm/shadow/set.c
>> +++ b/xen/arch/x86/mm/shadow/set.c
>> @@ -94,6 +94,22 @@ shadow_get_page_from_l1e(shadow_l1e_t sl
>>      ASSERT(!sh_l1e_is_magic(sl1e));
>>      ASSERT(shadow_mode_refcounts(d));
>>  
>> +    /*
>> +     * VMX'es APIC access MFN is just a surrogate page.  It doesn't actually
>> +     * get accessed, and hence there's no need to refcount it (and refcounting
>> +     * would fail, due to the page having no owner).
>> +     */
>> +    if ( mfn_valid(mfn = shadow_l1e_get_mfn(sl1e)) )
> 
> Would it be better to check specifically for mfn == apic_access_mfn
> (and apic_access_mfn != 0, I guess)?

Roger did ask about the same - I neither want to expose apic_access_mfn
outside its CU, nor do I want to introduce an accessor function. Both
feel like layering violations to me.

>  If we want this behaviour for
> for all un-owned PGC_extra MFNs it would be good to explain that in the
> comments.

This is hard to tell without knowing which (or even if) further such
PGC_extra pages will appear. Hence any comment to that effect would be
guesswork at best. Of course I can add e.g. "Other pages with the same
properties would be treated the same", if that's what you're after?

Jan


  reply	other threads:[~2021-04-19 11:25 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-22 10:55 [PATCH v3 0/2] VMX: apic access page handling adjustments Jan Beulich
2021-02-22 10:56 ` [PATCH v3 1/2][4.15] VMX: delay p2m insertion of APIC access page Jan Beulich
2021-02-22 11:25   ` Ian Jackson
2021-02-22 14:05     ` Jan Beulich
2021-02-22 17:17       ` Ian Jackson
2021-02-22 12:15   ` Roger Pau Monné
2021-02-25  8:44   ` Jan Beulich
2021-02-26  7:06     ` Tian, Kevin
2021-02-22 10:57 ` [PATCH v3 2/2] VMX: use a single, global " Jan Beulich
2021-03-01  2:34   ` Tian, Kevin
2021-03-01  8:18     ` Jan Beulich
2021-04-12 10:40 ` [PATCH v4] " Jan Beulich
2021-04-12 15:31   ` Roger Pau Monné
2021-04-13  9:24     ` Jan Beulich
2021-04-13 10:18       ` Roger Pau Monné
2021-04-13 12:03         ` Jan Beulich
2021-04-13 13:03           ` Roger Pau Monné
2021-04-17 19:24   ` Tim Deegan
2021-04-19 11:25     ` Jan Beulich [this message]
2021-04-22  7:42       ` Tim Deegan
2021-04-22  9:38         ` Jan Beulich
2021-04-22 15:05           ` Tim Deegan
2021-04-23 10:51 ` [PATCH v4 0/3] VMX APIC access page and shadow adjustments Jan Beulich
2021-04-23 10:52   ` [PATCH v4 1/3] VMX: use a single, global APIC access page Jan Beulich
2021-04-23 14:17     ` Roger Pau Monné
2021-04-23 14:42       ` Jan Beulich
2021-04-26 17:55         ` Tim Deegan
2021-04-25  1:27     ` Tian, Kevin
2021-04-26 17:53     ` Tim Deegan
2021-04-23 10:53   ` [PATCH v4 2/3] x86/shadow: re-use variables in shadow_get_page_from_l1e() Jan Beulich
2021-04-23 10:54   ` [PATCH v4 3/3] x86/shadow: streamline shadow_get_page_from_l1e() Jan Beulich
2021-04-23 11:00   ` Really v5 (was: [PATCH v4 0/3] VMX APIC access page and shadow adjustments) Jan Beulich

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=d698a29e-fb4e-a650-9e48-ad209e64c800@suse.com \
    --to=jbeulich@suse.com \
    --cc=andrew.cooper3@citrix.com \
    --cc=george.dunlap@citrix.com \
    --cc=jun.nakajima@intel.com \
    --cc=kevin.tian@intel.com \
    --cc=roger.pau@citrix.com \
    --cc=tim@xen.org \
    --cc=wl@xen.org \
    --cc=xen-devel@lists.xenproject.org \
    --subject='Re: [PATCH v4] VMX: use a single, global APIC access page' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).