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SubjectRe: [PATCH RFC v1 06/18] x86/hyperv: allocate output arg pages if required
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Wei Liu <wei.liu@kernel.org> writes:

> On Tue, Sep 15, 2020 at 12:43:18PM +0000, Wei Liu wrote:
>> On Tue, Sep 15, 2020 at 12:16:58PM +0200, Vitaly Kuznetsov wrote:
>> > Wei Liu <wei.liu@kernel.org> writes:
>> >
>> > > When Linux runs as the root partition, it will need to make hypercalls
>> > > which return data from the hypervisor.
>> > >
>> > > Allocate pages for storing results when Linux runs as the root
>> > > partition.
>> > >
>> > > Signed-off-by: Lillian Grassin-Drake <ligrassi@microsoft.com>
>> > > Co-Developed-by: Lillian Grassin-Drake <ligrassi@microsoft.com>
>> > > Signed-off-by: Wei Liu <wei.liu@kernel.org>
>> > > ---
>> > > arch/x86/hyperv/hv_init.c | 45 +++++++++++++++++++++++++++++----
>> > > arch/x86/include/asm/mshyperv.h | 1 +
>> > > 2 files changed, 41 insertions(+), 5 deletions(-)
>> > >
>> > > diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
>> > > index cac8e4c56261..ebba4be4185d 100644
>> > > --- a/arch/x86/hyperv/hv_init.c
>> > > +++ b/arch/x86/hyperv/hv_init.c
>> > > @@ -45,6 +45,9 @@ EXPORT_SYMBOL_GPL(hv_vp_assist_page);
>> > > void __percpu **hyperv_pcpu_input_arg;
>> > > EXPORT_SYMBOL_GPL(hyperv_pcpu_input_arg);
>> > >
>> > > +void __percpu **hyperv_pcpu_output_arg;
>> > > +EXPORT_SYMBOL_GPL(hyperv_pcpu_output_arg);
>> > > +
>> > > u32 hv_max_vp_index;
>> > > EXPORT_SYMBOL_GPL(hv_max_vp_index);
>> > >
>> > > @@ -75,14 +78,29 @@ static int hv_cpu_init(unsigned int cpu)
>> > > u64 msr_vp_index;
>> > > struct hv_vp_assist_page **hvp = &hv_vp_assist_page[smp_processor_id()];
>> > > void **input_arg;
>> > > - struct page *pg;
>> > > + struct page *input_pg;
>> > >
>> > > input_arg = (void **)this_cpu_ptr(hyperv_pcpu_input_arg);
>> > > /* hv_cpu_init() can be called with IRQs disabled from hv_resume() */
>> > > - pg = alloc_page(irqs_disabled() ? GFP_ATOMIC : GFP_KERNEL);
>> > > - if (unlikely(!pg))
>> > > + input_pg = alloc_page(irqs_disabled() ? GFP_ATOMIC : GFP_KERNEL);
>> > > + if (unlikely(!input_pg))
>> > > return -ENOMEM;
>> > > - *input_arg = page_address(pg);
>> > > + *input_arg = page_address(input_pg);
>> > > +
>> > > + if (hv_root_partition) {
>> > > + struct page *output_pg;
>> > > + void **output_arg;
>> > > +
>> > > + output_pg = alloc_page(irqs_disabled() ? GFP_ATOMIC :
>> > > GFP_KERNEL);
>> >
>> > To simplify the code, can we just rename 'input_arg' to 'hypercall_args'
>> > and do alloc_pages(rqs_disabled() ? GFP_ATOMIC : GFP_KERNEL, 1) to
>> > allocate two pages above?
>>
>> It should be doable, but I need to code it up and test it to be 100%
>> sure.
>>
>
> I switch to alloc_pages to allocate an order of 2 page if necessary, but
> I keep input_arg and output_arg separate because I want to avoid code
> churn in other places.
>

My idea was that we're free to choose how to use these pages, e.g. with
two pages allocated we can now do a hypercall which takes two pages of
input and produces no output other than the return value. This doesn't
contradict your suggestion to keep input_arg/output_arg as these are
just pointers to the continuous space, we can still do the trick.

I don't feel strong about this and you probably have more patches in
your stash, no need for massive re-work I believe.

--
Vitaly

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