Re: [Xen-devel] [PATCH v2 3/3] x86/hyperv: L0 assisted TLB flush

[Michael Kelley <mikelley@xxxxxxxxxxxxx>]

From: Wei Liu <wei.liu.xen@xxxxxxxxx> On Behalf Of Wei Liu Sent: Friday, 
February 14, 2020 4:35 AM
> 
> Implement L0 assisted TLB flush for Xen on Hyper-V. It takes advantage
> of several hypercalls:
> 
>  * HVCALL_FLUSH_VIRTUAL_ADDRESS_LIST
>  * HVCALL_FLUSH_VIRTUAL_ADDRESS_LIST_EX
>  * HVCALL_FLUSH_VIRTUAL_ADDRESS_SPACE
>  * HVCALL_FLUSH_VIRTUAL_ADDRESS_SPACE_EX
> 
> Pick the most efficient hypercalls available.
> 
> Signed-off-by: Wei Liu <liuwe@xxxxxxxxxxxxx>
> ---
> v2:
> 1. Address Roger and Jan's comments re types etc.
> 2. Fix pointer arithmetic.
> 3. Misc improvement to code.
> ---
>  xen/arch/x86/guest/hyperv/Makefile  |   1 +
>  xen/arch/x86/guest/hyperv/private.h |   9 ++
>  xen/arch/x86/guest/hyperv/tlb.c     | 172 +++++++++++++++++++++++++++-
>  xen/arch/x86/guest/hyperv/util.c    |  74 ++++++++++++
>  4 files changed, 255 insertions(+), 1 deletion(-)
>  create mode 100644 xen/arch/x86/guest/hyperv/util.c
> 
> diff --git a/xen/arch/x86/guest/hyperv/Makefile 
> b/xen/arch/x86/guest/hyperv/Makefile
> index 18902c33e9..0e39410968 100644
> --- a/xen/arch/x86/guest/hyperv/Makefile
> +++ b/xen/arch/x86/guest/hyperv/Makefile
> @@ -1,2 +1,3 @@
>  obj-y += hyperv.o
>  obj-y += tlb.o
> +obj-y += util.o
> diff --git a/xen/arch/x86/guest/hyperv/private.h 
> b/xen/arch/x86/guest/hyperv/private.h
> index 509bedaafa..79a77930a0 100644
> --- a/xen/arch/x86/guest/hyperv/private.h
> +++ b/xen/arch/x86/guest/hyperv/private.h
> @@ -24,12 +24,21 @@
> 
>  #include <xen/cpumask.h>
>  #include <xen/percpu.h>
> +#include <xen/types.h>
> 
>  DECLARE_PER_CPU(void *, hv_input_page);
>  DECLARE_PER_CPU(void *, hv_vp_assist);
>  DECLARE_PER_CPU(unsigned int, hv_vp_index);
> 
> +static inline unsigned int hv_vp_index(unsigned int cpu)
> +{
> +    return per_cpu(hv_vp_index, cpu);
> +}
> +
>  int hyperv_flush_tlb(const cpumask_t *mask, const void *va,
>                       unsigned int flags);
> 
> +/* Returns number of banks, -ev if error */
> +int cpumask_to_vpset(struct hv_vpset *vpset, const cpumask_t *mask);
> +
>  #endif /* __XEN_HYPERV_PRIVIATE_H__  */
> diff --git a/xen/arch/x86/guest/hyperv/tlb.c b/xen/arch/x86/guest/hyperv/tlb.c
> index 48f527229e..f68e14f151 100644
> --- a/xen/arch/x86/guest/hyperv/tlb.c
> +++ b/xen/arch/x86/guest/hyperv/tlb.c
> @@ -19,15 +19,185 @@
>   * Copyright (c) 2020 Microsoft.
>   */
> 
> +#include <xen/cpu.h>
>  #include <xen/cpumask.h>
>  #include <xen/errno.h>
> 
> +#include <asm/guest/hyperv.h>
> +#include <asm/guest/hyperv-hcall.h>
> +#include <asm/guest/hyperv-tlfs.h>
> +
>  #include "private.h"
> 
> +/*
> + * It is possible to encode up to 4096 pages using the lower 12 bits
> + * in an element of gva_list
> + */
> +#define HV_TLB_FLUSH_UNIT (4096 * PAGE_SIZE)
> +
> +static unsigned int fill_gva_list(uint64_t *gva_list, const void *va,
> +                                  unsigned int order)
> +{
> +    unsigned long start = (unsigned long)va;
> +    unsigned long end = start + (PAGE_SIZE << order) - 1;
> +    unsigned int n = 0;
> +
> +    do {
> +        unsigned long remain = end - start;

The calculated value here isn't actually the remaining bytes in the
range to flush -- it's one less than the remaining bytes in the range
to flush because of the -1 in the calculation of 'end'.   That difference
will mess up the comparison below against HV_TLB_FLUSH_UNIT
in the case that there are exactly 4096 page remaining to be
flushed.  It should take the "=" case, but won't.  Also, the
'-1' in 'remain - 1' in the else clause becomes unneeded, and
the 'start = end' assignment then propagates the error.

In the parallel code in Linux, if you follow the call sequence to get to
fill_gav_list(), the 'end' argument is really the address of the first byte
of the first page that isn't in the flush range (i.e., one beyond the true
'end') and so is a bit misnamed.

I think the calculation of 'end' should drop the -1, and perhaps 'end'
should be renamed.

Michael

> +
> +        gva_list[n] = start & PAGE_MASK;
> +
> +        /*
> +         * Use lower 12 bits to encode the number of additional pages
> +         * to flush
> +         */
> +        if ( remain >= HV_TLB_FLUSH_UNIT )
> +        {
> +            gva_list[n] |= ~PAGE_MASK;
> +            start += HV_TLB_FLUSH_UNIT;
> +        }
> +        else if ( remain )
> +        {
> +            gva_list[n] |= (remain - 1) >> PAGE_SHIFT;
> +            start = end;
> +        }
> +
> +        n++;
> +    } while ( start < end );
> +
> +    return n;
> +}
> +


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