"Michael Kelley (EOSG)" <[email protected]> writes:
>> -----Original Message----- >> From: [email protected] >> <[email protected]> On Behalf >> Of Vitaly Kuznetsov >> Sent: Friday, June 15, 2018 9:30 AM >> To: [email protected] >> Cc: [email protected]; [email protected]; KY Srinivasan >> <[email protected]>; Haiyang Zhang <[email protected]>; Stephen >> Hemminger >> <[email protected]>; Thomas Gleixner <[email protected]>; Ingo Molnar >> <[email protected]>; H. Peter Anvin <[email protected]>; Tianyu Lan >> <[email protected]> >> Subject: [PATCH] x86/hyper-v: use cheaper >> HVCALL_FLUSH_VIRTUAL_ADDRESS_{LIST,SPACE} >> hypercalls when possible >> >> While working on Hyper-V style PV TLB flush support in KVM I noticed that >> real Windows guests use TLB flush hypercall in a somewhat smarter way: when >> the flush needs to be performed on a subset of first 64 vCPUs or on all >> present vCPUs Windows avoids more expensive hypercalls which support >> sparse CPU sets and uses their 'cheap' counterparts. This means that >> HV_X64_EX_PROCESSOR_MASKS_RECOMMENDED name is actually a misnomer: EX >> hypercalls (which support sparse CPU sets) are "available", not >> "recommended". This makes sense as they are actually harder to parse. >> >> Nothing stops us from being equally 'smart' in Linux too. Switch to >> doing cheaper hypercalls whenever possible. >> >> Signed-off-by: Vitaly Kuznetsov <[email protected]> >> --- > > This is a good idea. We should probably do the same with the hypercalls for > sending > IPIs -- try the simpler version first and move to the more complex _EX > version only > if necessary. > > A complication: We've recently found a problem with the code for doing IPI > hypercalls, and the bug affects the TLB flush code as well. As secondary CPUs > are started, there's a window of time where the hv_vp_index entry for a > secondary CPU is uninitialized. We are seeing IPIs happening in that window, > and > the IPI hypercall code uses the uninitialized hv_vp_index entry. Same thing > could > happen with the TLB flush hypercall code. I didn't actually see any > occurrences of > the TLB case in my tracing, but we should fix it anyway in case a TLB flush > gets > added at some point in the future. > > KY has a patch coming. In the patch, hv_cpu_number_to_vp_number() > and cpumask_to_vpset() can both return U32_MAX if they encounter an > uninitialized hv_vp_index entry, and the code needs to be able to bail out to > the native functions for that particular IPI or TLB flush operation. Once the > initialization of secondary CPUs is complete, the uninitialized situation > won't > happen again, and the hypercall path will always be used. Sure, with TLB flush we can always fall back to doing it natively (by sending IPIs). > > We'll need to coordinate on these patches. Be aware that the IPI flavor of > the > bug is currently causing random failures when booting 4.18 RC1 on Hyper-V VMs > with large vCPU counts. Thanks for the heads up! This particular patch is just an optimization so there's no rush, IPI fix is definitely more important. > > Reviewed-by: Michael Kelley <[email protected]> Thanks! -- Vitaly _______________________________________________ devel mailing list [email protected] http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
