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Date: Thu, 23 Sep 2021 13:43:32 +0800 From: Lu Baolu <baolu.lu@...ux.intel.com> To: Fenghua Yu <fenghua.yu@...el.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Peter Zijlstra <peterz@...radead.org>, Andy Lutomirski <luto@...nel.org>, Dave Hansen <dave.hansen@...el.com>, Tony Luck <tony.luck@...el.com>, Joerg Roedel <joro@...tes.org>, Josh Poimboeuf <jpoimboe@...hat.com>, Dave Jiang <dave.jiang@...el.com>, Jacob Jun Pan <jacob.jun.pan@...el.com>, Ashok Raj <ashok.raj@...el.com>, Ravi V Shankar <ravi.v.shankar@...el.com> Cc: baolu.lu@...ux.intel.com, iommu@...ts.linux-foundation.org, x86 <x86@...nel.org>, linux-kernel <linux-kernel@...r.kernel.org> Subject: Re: [PATCH 5/8] x86/mmu: Add mm-based PASID refcounting Hi Fenghua, On 9/21/21 3:23 AM, Fenghua Yu wrote: > PASIDs are fundamentally hardware resources in a shared address space. > There is a limited number of them to use ENQCMD on shared workqueue. > They must be shared and managed. They can not, for instance, be > statically allocated to processes. > > Free PASID eagerly by sending IPIs in unbind was disabled due to locking > and other issues in commit 9bfecd058339 ("x86/cpufeatures: Force disable > X86_FEATURE_ENQCMD and remove update_pasid()"). > > Lazy PASID free is implemented in order to re-enable the ENQCMD feature. > PASIDs are currently reference counted and are centered around device > usage. To support lazy PASID free, reference counts are tracked in the > following scenarios: > > 1. The PASID's reference count is initialized as 1 when the PASID is first > allocated in bind. This is already implemented. > 2. A reference is taken when a device is bound to the mm and dropped > when the device is unbound from the mm. This reference tracks device > usage of the PASID. This is already implemented. > 3. A reference is taken when a task's IA32_PASID MSR is initialized in > #GP fix up and dropped when the task exits. This reference tracks > the task usage of the PASID. It is implemented here. > > Once a PASID is allocated to an mm in bind, it's associated to the mm until > it's freed lazily when its reference count is dropped to zero in unbind or > exit(2). > > ENQCMD requires a valid IA32_PASID MSR with the PASID value and a valid > PASID table entry for the PASID. Lazy PASID free may cause the process > still has the valid PASID but the PASID table entry is removed in unbind. > In this case, workqueue submitted by ENQCMD cannot find the PASID table > entry and will generate a DMAR fault. > > Here is a more detailed explanation of the life cycle of a PASID: > > All processes start out without a PASID allocated (because fork(2) > clears the PASID in the child). > > A PASID is allocated on the first open of an accelerator device by > a call to: > iommu_sva_bind_device() > -> intel_svm_bind() > -> intel_svm_alloc_pasid() > -> iommu_sva_alloc_pasid() > -> ioasid_alloc() > > At this point mm->pasid for the process is initialized, the reference > count on that PASID is 1, but as yet no tasks within the process have > set up their MSR_IA32_PASID to be able to execute the ENQCMD instruction. > > When a task in the process does execute ENQCMD there is a #GP fault. > The Linux handler notes that the process has a PASID allocated, and > attempts to fix the #GP fault by initializing MSR_IA32_PASID for this > task. It also increments the reference count for the PASID. > > Additional threads in the task may also execute ENQCMD, and each > will add to the reference count of the PASID. > > Tasks within the process may open additional accelerator devices. > In this case the call to iommu_sva_bind_device() merely increments > the reference count for the PASID. Since all devices use the same > PASID (all are accessing the same address space). > > So the reference count on a PASID is the sum of the number of open > accelerator devices plus the number of threads that have tried to > execute ENQCMD. > > The reverse happens as a process gives up resources. Each call to > iommu_sva_unbind_device() will reduce the reference count on the > PASID. Each task in the process that had set up MSR_IA32_PASID will > reduce the reference count as it exits. > > When the reference count is dropped to 0 in either task exit or > unbind, the PASID will be freed. > > Signed-off-by: Fenghua Yu <fenghua.yu@...el.com> > Reviewed-by: Tony Luck <tony.luck@...el.com> > --- > arch/x86/include/asm/iommu.h | 6 +++++ > arch/x86/include/asm/mmu_context.h | 2 ++ > drivers/iommu/intel/svm.c | 39 ++++++++++++++++++++++++++++++ > 3 files changed, 47 insertions(+) > > diff --git a/arch/x86/include/asm/iommu.h b/arch/x86/include/asm/iommu.h > index 9c4bf9b0702f..d00f0a3f32fb 100644 > --- a/arch/x86/include/asm/iommu.h > +++ b/arch/x86/include/asm/iommu.h > @@ -28,4 +28,10 @@ arch_rmrr_sanity_check(struct acpi_dmar_reserved_memory *rmrr) > > bool __fixup_pasid_exception(void); > > +#ifdef CONFIG_INTEL_IOMMU_SVM > +void pasid_put(struct task_struct *tsk, struct mm_struct *mm); > +#else > +static inline void pasid_put(struct task_struct *tsk, struct mm_struct *mm) { } > +#endif > + > #endif /* _ASM_X86_IOMMU_H */ > diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h > index 27516046117a..3a2de87e98a9 100644 > --- a/arch/x86/include/asm/mmu_context.h > +++ b/arch/x86/include/asm/mmu_context.h > @@ -12,6 +12,7 @@ > #include <asm/tlbflush.h> > #include <asm/paravirt.h> > #include <asm/debugreg.h> > +#include <asm/iommu.h> > > extern atomic64_t last_mm_ctx_id; > > @@ -146,6 +147,7 @@ do { \ > #else > #define deactivate_mm(tsk, mm) \ > do { \ > + pasid_put(tsk, mm); \ > load_gs_index(0); \ > loadsegment(fs, 0); \ > } while (0) > diff --git a/drivers/iommu/intel/svm.c b/drivers/iommu/intel/svm.c > index ab65020019b6..8b6b8007ba2c 100644 > --- a/drivers/iommu/intel/svm.c > +++ b/drivers/iommu/intel/svm.c > @@ -1187,6 +1187,7 @@ int intel_svm_page_response(struct device *dev, > bool __fixup_pasid_exception(void) > { > u32 pasid; > + int ret; > > /* > * This function is called only when this #GP was triggered from user > @@ -1205,9 +1206,47 @@ bool __fixup_pasid_exception(void) > if (current->has_valid_pasid) > return false; > > + mutex_lock(&pasid_mutex); > + /* The mm's pasid has been allocated. Take a reference to it. */ > + ret = iommu_sva_alloc_pasid(current->mm, PASID_MIN, > + intel_pasid_max_id - 1); > + mutex_unlock(&pasid_mutex); > + if (ret) > + return false; > + > /* Fix up the MSR by the PASID in the mm. */ > fpu__pasid_write(pasid); > current->has_valid_pasid = 1; > > return true; > } > + > +/* > + * pasid_put - On task exit release a reference to the mm's PASID > + * and free the PASID if no more reference > + * @mm: the mm > + * > + * When the task exits, release a reference to the mm's PASID if it was > + * allocated and the IA32_PASID MSR was fixed up. > + * > + * If there is no reference, the PASID is freed and can be allocated to > + * any process later. > + */ > +void pasid_put(struct task_struct *tsk, struct mm_struct *mm) > +{ > + if (!cpu_feature_enabled(X86_FEATURE_ENQCMD)) > + return; > + > + /* > + * Nothing to do if this task doesn't have a reference to the PASID. > + */ > + if (tsk->has_valid_pasid) { > + mutex_lock(&pasid_mutex); > + /* > + * The PASID's reference was taken during fix up. Release it > + * now. If the reference count is 0, the PASID is freed. > + */ > + iommu_sva_free_pasid(mm); > + mutex_unlock(&pasid_mutex); > + } > +} > It looks odd that both __fixup_pasid_exception() and pasid_put() are defined in the vendor IOMMU driver, but get called in the arch/x86 code. Is it feasible to move these two helpers to the files where they are called? The IA32_PASID MSR fixup and release are not part of the IOMMU implementation. Best regards, baolu
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