From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Rafael J. Wysocki" Subject: Re: [PATCH RFC] ACPI: disable extra P_LVLx access on KVM Date: Mon, 23 Apr 2018 10:07:10 +0200 Message-ID: <1969294.enFTGV94N4@aspire.rjw.lan> References: <1522431741-4678-1-git-send-email-mst@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Return-path: In-Reply-To: <1522431741-4678-1-git-send-email-mst@redhat.com> Sender: linux-kernel-owner@vger.kernel.org To: "Michael S. Tsirkin" Cc: linux-kernel@vger.kernel.org, Paolo Bonzini , Radim =?utf-8?B?S3LEjW3DocWZ?= , kvm@vger.kernel.org, Len Brown , linux-acpi@vger.kernel.org List-Id: linux-acpi@vger.kernel.org On Friday, March 30, 2018 7:43:58 PM CEST Michael S. Tsirkin wrote: > As documented by commit b488f021 "ACPI: restore comment justifying > 'extra' P_LVLx access", Linux does an extra IO read after entering idle > because on (some) chipsets STPCLK# doesn't get asserted in time > to prevent further instruction processing. > > This can never be the case on KVM, and a timer read causes an expensive > VM exit in turn causing useless load on host system. Detect KVM and skip > the read. TODO: whitelist more hypervisors? > > Note: very lightly tested. Pls don't apply this yet, I am working on a > _CST implementation for KVM and will repost this without the RFC tag > when it's been tested properly. > > Posting now for early flames/feedback. > > Cc: Paolo Bonzini > Cc: Radim Krčmář > Cc: kvm@vger.kernel.org > Signed-off-by: Michael S. Tsirkin > --- > drivers/acpi/processor_idle.c | 7 +++++-- > 1 file changed, 5 insertions(+), 2 deletions(-) > > diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c > index abb559c..8ae28dc 100644 > --- a/drivers/acpi/processor_idle.c > +++ b/drivers/acpi/processor_idle.c > @@ -33,6 +33,7 @@ > #include > #include > #include > +#include > > /* > * Include the apic definitions for x86 to have the APIC timer related defines > @@ -665,7 +666,8 @@ static void __cpuidle acpi_idle_do_entry(struct acpi_processor_cx *cx) > /* Dummy wait op - must do something useless after P_LVL2 read > because chipsets cannot guarantee that STPCLK# signal > gets asserted in time to freeze execution properly. */ > - inl(acpi_gbl_FADT.xpm_timer_block.address); > + if (!kvm_para_available()) > + inl(acpi_gbl_FADT.xpm_timer_block.address); > } > } > > @@ -687,7 +689,8 @@ static int acpi_idle_play_dead(struct cpuidle_device *dev, int index) > else if (cx->entry_method == ACPI_CSTATE_SYSTEMIO) { > inb(cx->address); > /* See comment in acpi_idle_do_entry() */ > - inl(acpi_gbl_FADT.xpm_timer_block.address); > + if (!kvm_para_available()) > + inl(acpi_gbl_FADT.xpm_timer_block.address); > } else > return -ENODEV; > } > While I have no objections to this change from the ACPI side, I'd like someone from the KVM land to comment on this. Thanks, Rafael