From: James Morse <james.morse@arm.com> To: Vladimir Murzin <vladimir.murzin@arm.com> Cc: marc.zyngier@arm.com, catalin.marinas@arm.com, kbuild-all@01.org, kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH v2 1/3] arm64: mm: Support Common Not Private translations Date: Wed, 18 Oct 2017 16:00:14 +0100 [thread overview] Message-ID: <59E76C7E.2060108@arm.com> (raw) In-Reply-To: <1507724395-13735-2-git-send-email-vladimir.murzin@arm.com> Hi Vladimir, On 11/10/17 13:19, Vladimir Murzin wrote: > Common Not Private (CNP) is a feature of ARMv8.2 extension which > allows translation table entries to be shared between different PEs in > the same inner shareable domain, so the hardware can use this fact to > optimise the caching of such entries in the TLB. > > CNP occupies one bit in TTBRx_ELy and VTTBR_EL2, which advertises to > the hardware that the translation table entries pointed to by this > TTBR are the same as every PE in the same inner shareable domain for > which the equivalent TTBR also has CNP bit set. In case CNP bit is set > but TTBR does not point at the same translation table entries or a > given ASID and VMID, then the system is mis-configured, so the results > of translations are UNPREDICTABLE. > > This patch adds support for Common Not Private translations on > different exceptions levels: > > (1) For EL0 there are a few cases we need to care of changes in > TTBR0_EL1: > - a switch to idmap > - software emulated PAN > we rule out latter via Kconfig options and for the former we make > sure that CNP is set for non-zero ASIDs only. I've been looking at how CNP interacts with the asid allocator. I think we depend on a subtlety that wasn't obvious to me at first. Can you check I'm reading this properly: The ARM-ARM's 'D4.8.1 Use of ASIDs and VMIDs to reduce TLB maintenance requirements' reads as if you can only share a TLB entry if both CPUs are using that ASID at the same time: > When the value of a TTBR_ELx.CnP field is 1, (on CPU-A) > translation table entries pointed to by that TTBR_ELx are shared with all > other PEs in the Inner Shareable domain for which the following conditions > are met: > The corresponding TTBR_ELx.CnP field has the value 1. (CPU-B's corresponding TTBR right?) This would suggest CPU-A stops sharing its TLB entries for an asid when it changes asid by scheduling a new task. A single-threaded task would never benefit from CNP. We will depend on this behaviour when we re-use an asid that was previously used on a remote CPU that hasn't yet noticed the rollover and invalidated its TLB. > diff --git a/arch/arm64/kernel/suspend.c b/arch/arm64/kernel/suspend.c > index 1e3be90..f28c44a 100644 > --- a/arch/arm64/kernel/suspend.c > +++ b/arch/arm64/kernel/suspend.c > @@ -46,6 +46,9 @@ void notrace __cpu_suspend_exit(void) > */ > cpu_uninstall_idmap(); > + /* Restore CnP bit in TTBR1_EL1 */ > + cpu_replace_ttbr1(lm_alias(swapper_pg_dir)); Could you wrap this in system_supports_cnp(). Otherwise it replaces ttbr1 unnecessarily. This function is called with the idmap loaded, it seems unnecessary to remove it twice. You could refactor cpu_replace_ttbr1() to have a __version that is called with the idmap loaded, then call that before the cpu_uninstall_idmap() above. Thanks, James
WARNING: multiple messages have this Message-ID (diff)
From: james.morse@arm.com (James Morse) To: linux-arm-kernel@lists.infradead.org Subject: [PATCH v2 1/3] arm64: mm: Support Common Not Private translations Date: Wed, 18 Oct 2017 16:00:14 +0100 [thread overview] Message-ID: <59E76C7E.2060108@arm.com> (raw) In-Reply-To: <1507724395-13735-2-git-send-email-vladimir.murzin@arm.com> Hi Vladimir, On 11/10/17 13:19, Vladimir Murzin wrote: > Common Not Private (CNP) is a feature of ARMv8.2 extension which > allows translation table entries to be shared between different PEs in > the same inner shareable domain, so the hardware can use this fact to > optimise the caching of such entries in the TLB. > > CNP occupies one bit in TTBRx_ELy and VTTBR_EL2, which advertises to > the hardware that the translation table entries pointed to by this > TTBR are the same as every PE in the same inner shareable domain for > which the equivalent TTBR also has CNP bit set. In case CNP bit is set > but TTBR does not point at the same translation table entries or a > given ASID and VMID, then the system is mis-configured, so the results > of translations are UNPREDICTABLE. > > This patch adds support for Common Not Private translations on > different exceptions levels: > > (1) For EL0 there are a few cases we need to care of changes in > TTBR0_EL1: > - a switch to idmap > - software emulated PAN > we rule out latter via Kconfig options and for the former we make > sure that CNP is set for non-zero ASIDs only. I've been looking at how CNP interacts with the asid allocator. I think we depend on a subtlety that wasn't obvious to me at first. Can you check I'm reading this properly: The ARM-ARM's 'D4.8.1 Use of ASIDs and VMIDs to reduce TLB maintenance requirements' reads as if you can only share a TLB entry if both CPUs are using that ASID at the same time: > When the value of a TTBR_ELx.CnP field is 1, (on CPU-A) > translation table entries pointed to by that TTBR_ELx are shared with all > other PEs in the Inner Shareable domain for which the following conditions > are met: > The corresponding TTBR_ELx.CnP field has the value 1. (CPU-B's corresponding TTBR right?) This would suggest CPU-A stops sharing its TLB entries for an asid when it changes asid by scheduling a new task. A single-threaded task would never benefit from CNP. We will depend on this behaviour when we re-use an asid that was previously used on a remote CPU that hasn't yet noticed the rollover and invalidated its TLB. > diff --git a/arch/arm64/kernel/suspend.c b/arch/arm64/kernel/suspend.c > index 1e3be90..f28c44a 100644 > --- a/arch/arm64/kernel/suspend.c > +++ b/arch/arm64/kernel/suspend.c > @@ -46,6 +46,9 @@ void notrace __cpu_suspend_exit(void) > */ > cpu_uninstall_idmap(); > + /* Restore CnP bit in TTBR1_EL1 */ > + cpu_replace_ttbr1(lm_alias(swapper_pg_dir)); Could you wrap this in system_supports_cnp(). Otherwise it replaces ttbr1 unnecessarily. This function is called with the idmap loaded, it seems unnecessary to remove it twice. You could refactor cpu_replace_ttbr1() to have a __version that is called with the idmap loaded, then call that before the cpu_uninstall_idmap() above. Thanks, James
next prev parent reply other threads:[~2017-10-18 15:00 UTC|newest] Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top 2017-10-11 12:19 [PATCH v2 0/3] Support Common Not Private translations Vladimir Murzin 2017-10-11 12:19 ` Vladimir Murzin 2017-10-11 12:19 ` [PATCH v2 1/3] arm64: mm: " Vladimir Murzin 2017-10-11 12:19 ` Vladimir Murzin 2017-10-18 15:00 ` James Morse [this message] 2017-10-18 15:00 ` James Morse 2017-12-13 14:19 ` James Morse 2017-12-13 14:19 ` James Morse 2017-12-13 16:59 ` Vladimir Murzin 2017-12-13 16:59 ` Vladimir Murzin 2017-10-11 12:19 ` [PATCH v2 2/3] arm64: KVM: " Vladimir Murzin 2017-10-11 12:19 ` Vladimir Murzin 2017-10-11 12:19 ` [PATCH v2 3/3] arm64: Introduce command line parameter to disable CNP Vladimir Murzin 2017-10-11 12:19 ` Vladimir Murzin
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=59E76C7E.2060108@arm.com \ --to=james.morse@arm.com \ --cc=catalin.marinas@arm.com \ --cc=kbuild-all@01.org \ --cc=kvmarm@lists.cs.columbia.edu \ --cc=linux-arm-kernel@lists.infradead.org \ --cc=marc.zyngier@arm.com \ --cc=vladimir.murzin@arm.com \ /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: linkBe sure your reply has a Subject: header at the top and a blank line before the message body.
This is an external index of several public inboxes, see mirroring instructions on how to clone and mirror all data and code used by this external index.