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From:   "Michael Kelley (EOSG)" <Michael.H.Kelley@microsoft.com>
To:     Tianyu Lan <Tianyu.Lan@microsoft.com>
CC:     KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
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Subject: RE: [PATCH V2 1/5] X86/Hyper-V: Add flush
 HvFlushGuestPhysicalAddressSpace hypercall support
Thread-Topic: [PATCH V2 1/5] X86/Hyper-V: Add flush
 HvFlushGuestPhysicalAddressSpace hypercall support
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Date:   Tue, 10 Jul 2018 21:29:24 +0000
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From: Tianyu Lan <Tianyu.Lan@microsoft.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.
>=20
> Signed-off-by: Lan Tianyu <Tianyu.Lan@microsoft.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 =3D -EFAULT;
> +
> +	if (!hv_hypercall_pg)
> +		goto fault;
> +
> +	local_irq_save(flags);
> +
> +	flush_pcpu =3D (struct hv_guest_mapping_flush **)
> +		this_cpu_ptr(hyperv_pcpu_input_arg);
> +
> +	flush =3D *flush_pcpu;
> +
> +	if (unlikely(!flush)) {
> +		local_irq_restore(flags);
> +		goto fault;
> +	}
> +
> +	flush->address_space =3D as;
> +	flush->flags =3D 0;
> +
> +	status =3D 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 byte=
s
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 fo=
r
getting the per-cpu input arg or the code for local irq save/restore, so th=
e
code that is left is a lot faster and simpler.

Michael

> +	local_irq_restore(flags);
> +
> +	if (!(status & HV_HYPERCALL_RESULT_MASK))
> +		ret =3D 0;
> +
> +fault:
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(hyperv_flush_guest_mapping);