From: Jan Beulich <email@example.com> To: Tim Deegan <firstname.lastname@example.org> Cc: "email@example.com" <firstname.lastname@example.org>, "Andrew Cooper" <email@example.com>, "Wei Liu" <firstname.lastname@example.org>, "Roger Pau Monné" <email@example.com>, "George Dunlap" <firstname.lastname@example.org>, "Kevin Tian" <email@example.com>, "Jun Nakajima" <firstname.lastname@example.org> 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: <email@example.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
next prev parent 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
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