From: "Jan Beulich" <JBeulich@suse.com>
To: Juergen Gross <jgross@suse.com>
Cc: andrew.cooper3@citrix.com, Dario Faggioli <dfaggioli@suse.com>,
wei.liu2@citrix.com, xen-devel@lists.xenproject.org
Subject: Re: [PATCH v2 5/6] xen/x86: use flag byte for decision whether xen_cr3 is valid
Date: Thu, 08 Mar 2018 07:24:46 -0700 [thread overview]
Message-ID: <5AA155BE02000078001AFD89@prv-mh.provo.novell.com> (raw)
In-Reply-To: <20180302081403.16953-6-jgross@suse.com>
>>> On 02.03.18 at 09:14, <jgross@suse.com> wrote:
> This reduces the number of branches in interrupt handling and results
> in better performance (e.g. parallel make of the Xen hypervisor on my
> system was using about 3% less system time).
3% seems an awful lot for a single conditional branch on each of the
three affected entry paths.
> --- a/xen/arch/x86/domain.c
> +++ b/xen/arch/x86/domain.c
> @@ -1698,6 +1698,7 @@ void context_switch(struct vcpu *prev, struct vcpu *next)
> ASSERT(local_irq_is_enabled());
>
> get_cpu_info()->xen_cr3 = 0;
> + get_cpu_info()->use_xen_cr3 = false;
Don't you need this to be the other way around _and_ a barrier() in
between? As the context above shows, interrupts are enabled here
(and NMI/#MC can occur at any time anyway), so with the order
above it seems to me as if restore_all_xen might write zero into CR3.
While the ordering appears to be right elsewhere, the barrier() part
may apply to changes further down as well.
> @@ -523,18 +516,17 @@ ENTRY(common_interrupt)
>
> .Lintr_cr3_start:
> mov STACK_CPUINFO_FIELD(xen_cr3)(%r14), %rcx
> + mov STACK_CPUINFO_FIELD(use_xen_cr3)(%r14), %bl
> mov %rcx, %r15
> - neg %rcx
> + test %rcx, %rcx
> jz .Lintr_cr3_okay
> - jns .Lintr_cr3_load
> - mov %rcx, STACK_CPUINFO_FIELD(xen_cr3)(%r14)
> - neg %rcx
> -.Lintr_cr3_load:
> + movb $0, STACK_CPUINFO_FIELD(use_xen_cr3)(%r14)
> mov %rcx, %cr3
> xor %ecx, %ecx
> mov %rcx, STACK_CPUINFO_FIELD(xen_cr3)(%r14)
> testb $3, UREGS_cs(%rsp)
> cmovnz %rcx, %r15
> + cmovnz %cx, %bx
32-bit operation please.
> @@ -831,6 +820,7 @@ handle_ist_exception:
> * and copy the context to stack bottom.
> */
> xor %r15, %r15
> + xor %bl, %bl
Same here.
> @@ -68,6 +65,12 @@ struct cpu_info {
> */
> bool root_pgt_changed;
>
> + /*
> + * use_xen_cr3 is set in case the value of xen_cr3 is to be written into
> + * CR3 when entering the hypervisor.
> + */
> + bool use_xen_cr3;
When entering the hypervisor? Afaics the flag is evaluated only to
trigger the unlikely code in restore_all_xen, which is an exit path (as
the comment portion you remove from xen_cr3 also says).
Jan
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next prev parent reply other threads:[~2018-03-08 14:24 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-03-02 8:13 [PATCH v2 0/6] xen/x86: various XPTI speedups Juergen Gross
2018-03-02 8:13 ` [PATCH v2 1/6] x86/xpti: avoid copying L4 page table contents when possible Juergen Gross
2018-03-05 16:43 ` Jan Beulich
2018-03-08 11:59 ` Juergen Gross
2018-03-08 12:47 ` Jan Beulich
[not found] ` <5AA13EEA02000078001AFCAF@suse.com>
2018-03-08 13:03 ` Juergen Gross
[not found] ` <5A9D81DC02000078001AEB68@suse.com>
2018-03-06 7:01 ` Juergen Gross
2018-03-06 7:58 ` Jan Beulich
[not found] ` <5A9E583002000078001AED3A@suse.com>
2018-03-06 8:06 ` Juergen Gross
2018-03-06 8:17 ` Jan Beulich
2018-03-02 8:13 ` [PATCH v2 2/6] x86/xpti: don't flush TLB twice when switching to 64-bit pv context Juergen Gross
2018-03-05 16:49 ` Jan Beulich
[not found] ` <5A9D831F02000078001AEB7E@suse.com>
2018-03-06 7:02 ` Juergen Gross
2018-03-02 8:14 ` [PATCH v2 3/6] xen/x86: support per-domain flag for xpti Juergen Gross
2018-03-08 10:17 ` Jan Beulich
[not found] ` <5AA11BDE02000078001AFB92@suse.com>
2018-03-08 11:30 ` Juergen Gross
2018-03-08 12:49 ` Jan Beulich
[not found] ` <5AA13F7D02000078001AFCB3@suse.com>
2018-03-08 13:13 ` Juergen Gross
2018-03-02 8:14 ` [PATCH v2 4/6] xen/x86: disable global pages for domains with XPTI active Juergen Gross
2018-03-02 11:03 ` Wei Liu
2018-03-02 11:30 ` Juergen Gross
2018-03-08 13:38 ` Jan Beulich
2018-03-09 3:01 ` Tian, Kevin
2018-03-09 5:23 ` Tian, Kevin
2018-03-09 8:34 ` Jan Beulich
[not found] ` <5AA2551002000078001B0116@suse.com>
2018-03-09 8:42 ` Juergen Gross
[not found] ` <5AA14AF302000078001AFD30@suse.com>
2018-03-08 14:05 ` Juergen Gross
2018-03-08 14:33 ` Jan Beulich
[not found] ` <5AA157E002000078001AFDA4@suse.com>
2018-03-08 14:39 ` Juergen Gross
2018-03-08 15:06 ` Jan Beulich
2018-03-09 14:40 ` Juergen Gross
2018-03-09 15:30 ` Jan Beulich
2018-03-02 8:14 ` [PATCH v2 5/6] xen/x86: use flag byte for decision whether xen_cr3 is valid Juergen Gross
2018-03-08 14:24 ` Jan Beulich [this message]
[not found] ` <5AA155BE02000078001AFD89@suse.com>
2018-03-08 14:28 ` Juergen Gross
2018-03-02 8:14 ` [PATCH v2 6/6] xen/x86: use PCID feature for XPTI Juergen Gross
2018-03-08 15:27 ` Jan Beulich
2018-03-05 16:20 ` [PATCH v2 0/6] xen/x86: various XPTI speedups Dario Faggioli
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