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[60.242.11.44]) by smtp.gmail.com with ESMTPSA id c9sm7233627pjr.44.2021.02.04.22.28.41 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 04 Feb 2021 22:28:41 -0800 (PST) Date: Fri, 05 Feb 2021 16:28:35 +1000 From: Nicholas Piggin Subject: Re: [PATCH v2 1/1] powerpc/kvm: Save Timebase Offset to fix sched_clock() while running guest code. To: "Aneesh Kumar K.V" , Athira Rajeev , Benjamin Herrenschmidt , Christophe Leroy , Frederic Weisbecker , Geert Uytterhoeven , Jordan Niethe , Leonardo Bras , Michael Ellerman , Paul Mackerras , Thomas Gleixner References: <20210205060643.233481-1-leobras.c@gmail.com> In-Reply-To: <20210205060643.233481-1-leobras.c@gmail.com> MIME-Version: 1.0 Message-Id: <1612506268.6rrvx34gzu.astroid@bobo.none> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, kvm-ppc@vger.kernel.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" Excerpts from Leonardo Bras's message of February 5, 2021 4:06 pm: > Before guest entry, TBU40 register is changed to reflect guest timebase. > After exitting guest, the register is reverted to it's original value. >=20 > If one tries to get the timestamp from host between those changes, it > will present an incorrect value. >=20 > An example would be trying to add a tracepoint in > kvmppc_guest_entry_inject_int(), which depending on last tracepoint > acquired could actually cause the host to crash. >=20 > Save the Timebase Offset to PACA and use it on sched_clock() to always > get the correct timestamp. Ouch. Not sure how reasonable it is to half switch into guest registers=20 and expect to call into the wider kernel, fixing things up as we go.=20 What if mftb is used in other places? Especially as it doesn't seem like there is a reason that function _has_ to be called after the timebase is switched to guest, that's just how=20 the code is structured. As a local hack to work out a bug okay. If you really need it upstream=20 could you put it under a debug config option? Thanks, Nick > Signed-off-by: Leonardo Bras > Suggested-by: Paul Mackerras > --- > Changes since v1: > - Subtracts offset only when CONFIG_KVM_BOOK3S_HANDLER and > CONFIG_PPC_BOOK3S_64 are defined. > --- > arch/powerpc/include/asm/kvm_book3s_asm.h | 1 + > arch/powerpc/kernel/asm-offsets.c | 1 + > arch/powerpc/kernel/time.c | 8 +++++++- > arch/powerpc/kvm/book3s_hv.c | 2 ++ > arch/powerpc/kvm/book3s_hv_rmhandlers.S | 2 ++ > 5 files changed, 13 insertions(+), 1 deletion(-) >=20 > diff --git a/arch/powerpc/include/asm/kvm_book3s_asm.h b/arch/powerpc/inc= lude/asm/kvm_book3s_asm.h > index 078f4648ea27..e2c12a10eed2 100644 > --- a/arch/powerpc/include/asm/kvm_book3s_asm.h > +++ b/arch/powerpc/include/asm/kvm_book3s_asm.h > @@ -131,6 +131,7 @@ struct kvmppc_host_state { > u64 cfar; > u64 ppr; > u64 host_fscr; > + u64 tb_offset; /* Timebase offset: keeps correct timebase while on gue= st */ > #endif > }; > =20 > diff --git a/arch/powerpc/kernel/asm-offsets.c b/arch/powerpc/kernel/asm-= offsets.c > index b12d7c049bfe..0beb8fdc6352 100644 > --- a/arch/powerpc/kernel/asm-offsets.c > +++ b/arch/powerpc/kernel/asm-offsets.c > @@ -706,6 +706,7 @@ int main(void) > HSTATE_FIELD(HSTATE_CFAR, cfar); > HSTATE_FIELD(HSTATE_PPR, ppr); > HSTATE_FIELD(HSTATE_HOST_FSCR, host_fscr); > + HSTATE_FIELD(HSTATE_TB_OFFSET, tb_offset); > #endif /* CONFIG_PPC_BOOK3S_64 */ > =20 > #else /* CONFIG_PPC_BOOK3S */ > diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c > index 67feb3524460..f27f0163792b 100644 > --- a/arch/powerpc/kernel/time.c > +++ b/arch/powerpc/kernel/time.c > @@ -699,7 +699,13 @@ EXPORT_SYMBOL_GPL(tb_to_ns); > */ > notrace unsigned long long sched_clock(void) > { > - return mulhdu(get_tb() - boot_tb, tb_to_ns_scale) << tb_to_ns_shift; > + u64 tb =3D get_tb() - boot_tb; > + > +#if defined(CONFIG_PPC_BOOK3S_64) && defined(CONFIG_KVM_BOOK3S_HANDLER) > + tb -=3D local_paca->kvm_hstate.tb_offset; > +#endif > + > + return mulhdu(tb, tb_to_ns_scale) << tb_to_ns_shift; > } > =20 > =20 > diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c > index b3731572295e..c08593c63353 100644 > --- a/arch/powerpc/kvm/book3s_hv.c > +++ b/arch/powerpc/kvm/book3s_hv.c > @@ -3491,6 +3491,7 @@ static int kvmhv_load_hv_regs_and_go(struct kvm_vcp= u *vcpu, u64 time_limit, > if ((tb & 0xffffff) < (new_tb & 0xffffff)) > mtspr(SPRN_TBU40, new_tb + 0x1000000); > vc->tb_offset_applied =3D vc->tb_offset; > + local_paca->kvm_hstate.tb_offset =3D vc->tb_offset; > } > =20 > if (vc->pcr) > @@ -3594,6 +3595,7 @@ static int kvmhv_load_hv_regs_and_go(struct kvm_vcp= u *vcpu, u64 time_limit, > if ((tb & 0xffffff) < (new_tb & 0xffffff)) > mtspr(SPRN_TBU40, new_tb + 0x1000000); > vc->tb_offset_applied =3D 0; > + local_paca->kvm_hstate.tb_offset =3D 0; > } > =20 > mtspr(SPRN_HDEC, 0x7fffffff); > diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S b/arch/powerpc/kvm/b= ook3s_hv_rmhandlers.S > index b73140607875..8f7a9f7f4ee6 100644 > --- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S > +++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S > @@ -632,6 +632,7 @@ END_FTR_SECTION_IFCLR(CPU_FTR_ARCH_300) > cmpdi r8,0 > beq 37f > std r8, VCORE_TB_OFFSET_APPL(r5) > + std r8, HSTATE_TB_OFFSET(r13) > mftb r6 /* current host timebase */ > add r8,r8,r6 > mtspr SPRN_TBU40,r8 /* update upper 40 bits */ > @@ -1907,6 +1908,7 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S) > beq 17f > li r0, 0 > std r0, VCORE_TB_OFFSET_APPL(r5) > + std r0, HSTATE_TB_OFFSET(r13) > mftb r6 /* current guest timebase */ > subf r8,r8,r6 > mtspr SPRN_TBU40,r8 /* update upper 40 bits */ > --=20 > 2.29.2 >=20 >=20