On Wed, Sep 28, 2022 at 05:56:40PM +0800, Zhao Liu wrote: > From: Zhao Liu <zhao1.liu@xxxxxxxxx> > > kmap() is being deprecated in favor of kmap_local_page()[1]. > > There are two main problems with kmap(): (1) It comes with an overhead as > mapping space is restricted and protected by a global lock for > synchronization and (2) it also requires global TLB invalidation when the > kmap's pool wraps and it might block when the mapping space is fully > utilized until a slot becomes available. > > With kmap_local_page() the mappings are per thread, CPU local, can take > page faults, and can be called from any context (including interrupts). > It is faster than kmap() in kernels with HIGHMEM enabled. Furthermore, > the tasks can be preempted and, when they are scheduled to run again, the > kernel virtual addresses are restored and are still valid. > > In the fuction hyperv_init() of hyperv/hv_init.c, the mapping is used in a > single thread and is short live. So, in this case, it's safe to simply use > kmap_local_page() to create mapping, and this avoids the wasted cost of > kmap() for global synchronization. > The kmap call in that function is not performance critical in any way, and at this point in the initialization process I don't expect there to be any contention, so the downside of kmap is not really a concern here. That being said, kmap getting deprecated is a good enough reason to switch to kmap_local_page. And I appreciate this well-written, well-reasoned commit message. I will apply it to hyperv-next later -- I doubt people will object to this change, but just in case. Thanks, Wei. > In addtion, the fuction hyperv_init() checks if kmap() fails by BUG_ON(). > From the original discussion[2], the BUG_ON() here is just used to > explicitly panic NULL pointer. So still keep the BUG_ON() in place to check > if kmap_local_page() fails. Based on this consideration, memcpy_to_page() > is not selected here but only kmap_local_page() is used. > > Therefore, replace kmap() with kmap_local_page() in hyperv/hv_init.c. > > [1]: https://lore.kernel.org/all/20220813220034.806698-1-ira.weiny@xxxxxxxxx > [2]: https://lore.kernel.org/lkml/20200915103710.cqmdvzh5lys4wsqo@liuwe-devbox-debian-v2/ > > Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxx> > Suggested-by: Ira Weiny <ira.weiny@xxxxxxxxx> > Suggested-by: Fabio M. De Francesco <fmdefrancesco@xxxxxxxxx> > Signed-off-by: Zhao Liu <zhao1.liu@xxxxxxxxx> > > --- > Suggested by credits. > Dave: Referred to his comments about whether kmap() can fail and the > suggestion to keep BUG_ON() in place. > Ira: Referred to his task documentation and review comments about > keeping BUG_ON() for kmap_local_page(). > Fabio: Stole some of his boiler plate commit message. > --- > arch/x86/hyperv/hv_init.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c > index 3de6d8b53367..72fe46eb183f 100644 > --- a/arch/x86/hyperv/hv_init.c > +++ b/arch/x86/hyperv/hv_init.c > @@ -459,13 +459,13 @@ void __init hyperv_init(void) > wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64); > > pg = vmalloc_to_page(hv_hypercall_pg); > - dst = kmap(pg); > + dst = kmap_local_page(pg); > src = memremap(hypercall_msr.guest_physical_address << PAGE_SHIFT, PAGE_SIZE, > MEMREMAP_WB); > BUG_ON(!(src && dst)); > memcpy(dst, src, HV_HYP_PAGE_SIZE); > memunmap(src); > - kunmap(pg); > + kunmap_local(dst); > } else { > hypercall_msr.guest_physical_address = vmalloc_to_pfn(hv_hypercall_pg); > wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64); > -- > 2.34.1 >
