linux-kernel - RE: [PATCH V2 1/5] X86/Hyper-V: Add flush HvFlushGuestPhysicalAddressSpace hypercall support

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Message-ID: <SN6PR2101MB11202EAE0D5C623EC0CF8273DC5B0@SN6PR2101MB1120.namprd21.prod.outlook.com>
Date:   Tue, 10 Jul 2018 21:29:24 +0000
From:   "Michael Kelley (EOSG)" <Michael.H.Kelley@...rosoft.com>
To:     Tianyu Lan <Tianyu.Lan@...rosoft.com>
CC:     KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "hpa@...or.com" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>
Subject: RE: [PATCH V2 1/5] X86/Hyper-V: Add flush
 HvFlushGuestPhysicalAddressSpace hypercall support

From: Tianyu Lan <Tianyu.Lan@...rosoft.com> Monday, July 9, 2018 2:03 AM
> Hyper-V supports a pv hypercall HvFlushGuestPhysicalAddressSpace to
> flush nested VM address space mapping in l1 hypervisor and it's to
> reduce overhead of flushing ept tlb among vcpus. This patch is to
> implement it.
> 
> Signed-off-by: Lan Tianyu <Tianyu.Lan@...rosoft.com>
> ---
>  arch/x86/hyperv/Makefile           |  2 +-
>  arch/x86/hyperv/nested.c           | 64 ++++++++++++++++++++++++++++++++++++++
>  arch/x86/include/asm/hyperv-tlfs.h |  8 +++++
>  arch/x86/include/asm/mshyperv.h    |  2 ++
>  4 files changed, 75 insertions(+), 1 deletion(-)
>  create mode 100644 arch/x86/hyperv/nested.c
> +#include <linux/types.h>
> +#include <asm/hyperv-tlfs.h>
> +#include <asm/mshyperv.h>
> +#include <asm/tlbflush.h>
> +
> +int hyperv_flush_guest_mapping(u64 as)
> +{
> +	struct hv_guest_mapping_flush **flush_pcpu;
> +	struct hv_guest_mapping_flush *flush;
> +	u64 status;
> +	unsigned long flags;
> +	int ret = -EFAULT;
> +
> +	if (!hv_hypercall_pg)
> +		goto fault;
> +
> +	local_irq_save(flags);
> +
> +	flush_pcpu = (struct hv_guest_mapping_flush **)
> +		this_cpu_ptr(hyperv_pcpu_input_arg);
> +
> +	flush = *flush_pcpu;
> +
> +	if (unlikely(!flush)) {
> +		local_irq_restore(flags);
> +		goto fault;
> +	}
> +
> +	flush->address_space = as;
> +	flush->flags = 0;
> +
> +	status = hv_do_hypercall(HVCALL_FLUSH_GUEST_PHYSICAL_ADDRESS_SPACE,
> +				 flush, NULL);

Did you consider using a "fast" hypercall?  Unless there's some reason I'm
not aware of, a "fast" hypercall would be perfect here as there are 16 bytes
of input and no output. Vitaly recently added hv_do_fast_hypercall16()
in the linux-next tree. See __send_ipi_mask() in hv_apic.c in linux-next
for an example of usage.  With a fast hypercall, you don't need the code for
getting the per-cpu input arg or the code for local irq save/restore, so the
code that is left is a lot faster and simpler.

Michael

> +	local_irq_restore(flags);
> +
> +	if (!(status & HV_HYPERCALL_RESULT_MASK))
> +		ret = 0;
> +
> +fault:
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(hyperv_flush_guest_mapping);