Try to allocate larger order pages.
With some test memory program stressing TLB (many small random
memory accesses) you can get 15% performance improves.
On the first memory iteration the sender is currently sending
memory in 4mb aligned chunks which allows the receiver to
allocate most pages as 2mb superpages instead of single 4kb pages.
This works even for HVM where the first 2mb contains some holes.
This change does not handle 1gb superpages as this will require
change in the protocol to preallocate space.
Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
---
Changes since v1:
- updated commit message and subject;
- change the implementation detecting possible 2mb pages inside
the packet sent allowing more 2mb superpages.
Changes since v2:
- change implementation simplifying detecting and allocations
of 2mb pages.
---
tools/libs/guest/xg_sr_restore.c | 45 +++++++++++++++++++++++++++++---
1 file changed, 42 insertions(+), 3 deletions(-)
diff --git a/tools/libs/guest/xg_sr_restore.c b/tools/libs/guest/xg_sr_restore.c
index 06231ca826..ea5a137612 100644
--- a/tools/libs/guest/xg_sr_restore.c
+++ b/tools/libs/guest/xg_sr_restore.c
@@ -129,6 +129,30 @@ static int pfn_set_populated(struct xc_sr_context *ctx, xen_pfn_t pfn)
return 0;
}
+#if defined(__i386__) || defined(__x86_64__)
+/* Order of the smallest superpage */
+#define SMALL_SUPERPAGE_ORDER 9
+#else
+#error Define SMALL_SUPERPAGE_ORDER for this platform
+#endif
+
+static bool populate_small_superpage(struct xc_sr_context *ctx, xen_pfn_t pfn)
+{
+ xen_pfn_t mfn = pfn;
+
+ if ( xc_domain_populate_physmap_exact(
+ ctx->xch, ctx->domid, 1, SMALL_SUPERPAGE_ORDER, 0, &mfn) )
+ return false;
+
+ if ( mfn == INVALID_MFN )
+ return false;
+
+ for ( size_t i = 0; i < (1 << SMALL_SUPERPAGE_ORDER); ++i )
+ ctx->restore.ops.set_gfn(ctx, pfn + i, mfn + i);
+
+ return true;
+}
+
/*
* Given a set of pfns, obtain memory from Xen to fill the physmap for the
* unpopulated subset. If types is NULL, no page type checking is performed
@@ -142,6 +166,9 @@ int populate_pfns(struct xc_sr_context *ctx, unsigned int count,
*pfns = malloc(count * sizeof(*pfns));
unsigned int i, nr_pfns = 0;
int rc = -1;
+ xen_pfn_t prev = 0;
+ unsigned num_contiguous = 0;
+ xen_pfn_t mask = ~((~(xen_pfn_t)0) << SMALL_SUPERPAGE_ORDER);
if ( !mfns || !pfns )
{
@@ -152,14 +179,26 @@ int populate_pfns(struct xc_sr_context *ctx, unsigned int count,
for ( i = 0; i < count; ++i )
{
+ xen_pfn_t pfn = original_pfns[i];
+
if ( (!types || page_type_to_populate(types[i])) &&
- !pfn_is_populated(ctx, original_pfns[i]) )
+ !pfn_is_populated(ctx, pfn) )
{
- rc = pfn_set_populated(ctx, original_pfns[i]);
+ rc = pfn_set_populated(ctx, pfn);
if ( rc )
goto err;
- pfns[nr_pfns] = mfns[nr_pfns] = original_pfns[i];
+ pfns[nr_pfns] = mfns[nr_pfns] = pfn;
++nr_pfns;
+ if ( pfn != prev + 1 )
+ num_contiguous = 0;
+ num_contiguous++;
+ prev = pfn;
+ if ( num_contiguous > mask && (pfn & mask) == mask &&
+ populate_small_superpage(ctx, pfn - mask) )
+ {
+ nr_pfns -= mask + 1;
+ num_contiguous = 0;
+ }
}
}
--
2.43.0On 12/09/2025 10:57 am, Frediano Ziglio wrote:
> Try to allocate larger order pages.
> With some test memory program stressing TLB (many small random
> memory accesses) you can get 15% performance improves.
> On the first memory iteration the sender is currently sending
> memory in 4mb aligned chunks which allows the receiver to
> allocate most pages as 2mb superpages instead of single 4kb pages.
> This works even for HVM where the first 2mb contains some holes.
> This change does not handle 1gb superpages as this will require
> change in the protocol to preallocate space.
>
> Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com>
> Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Thanks, this is far easier to follow. A couple of minor things.
> ---
> Changes since v1:
> - updated commit message and subject;
> - change the implementation detecting possible 2mb pages inside
> the packet sent allowing more 2mb superpages.
>
> Changes since v2:
> - change implementation simplifying detecting and allocations
> of 2mb pages.
> ---
> tools/libs/guest/xg_sr_restore.c | 45 +++++++++++++++++++++++++++++---
> 1 file changed, 42 insertions(+), 3 deletions(-)
>
> diff --git a/tools/libs/guest/xg_sr_restore.c b/tools/libs/guest/xg_sr_restore.c
> index 06231ca826..ea5a137612 100644
> --- a/tools/libs/guest/xg_sr_restore.c
> +++ b/tools/libs/guest/xg_sr_restore.c
> @@ -129,6 +129,30 @@ static int pfn_set_populated(struct xc_sr_context *ctx, xen_pfn_t pfn)
> return 0;
> }
>
> +#if defined(__i386__) || defined(__x86_64__)
> +/* Order of the smallest superpage */
> +#define SMALL_SUPERPAGE_ORDER 9
> +#else
> +#error Define SMALL_SUPERPAGE_ORDER for this platform
> +#endif
> +
> +static bool populate_small_superpage(struct xc_sr_context *ctx, xen_pfn_t pfn)
I know the terminology is terrible (this work was what prompted some of
my clean-up attempts in Xen).
I think we want to s/pfn/gfn/ all across this function.
> +{
> + xen_pfn_t mfn = pfn;
> +
> + if ( xc_domain_populate_physmap_exact(
> + ctx->xch, ctx->domid, 1, SMALL_SUPERPAGE_ORDER, 0, &mfn) )
This needs a comment.
/* XENMEM_populate_physmap has no coherent error semantics. Assume a
failure here is ENOMEM, and fall back to allocating small pages. */
(Yes, the physmap hypercalls are insane. The only error feedback is "I
completed this many before something went wrong", and libxenctrl chooses
EBUSY for want of anything better.)
> + return false;
> +
> + if ( mfn == INVALID_MFN )
> + return false;
> +
> + for ( size_t i = 0; i < (1 << SMALL_SUPERPAGE_ORDER); ++i )
> + ctx->restore.ops.set_gfn(ctx, pfn + i, mfn + i);
> +
> + return true;
> +}
> +
> /*
> * Given a set of pfns, obtain memory from Xen to fill the physmap for the
> * unpopulated subset. If types is NULL, no page type checking is performed
> @@ -142,6 +166,9 @@ int populate_pfns(struct xc_sr_context *ctx, unsigned int count,
> *pfns = malloc(count * sizeof(*pfns));
> unsigned int i, nr_pfns = 0;
> int rc = -1;
> + xen_pfn_t prev = 0;
> + unsigned num_contiguous = 0;
> + xen_pfn_t mask = ~((~(xen_pfn_t)0) << SMALL_SUPERPAGE_ORDER);
(1ULL << SMALL_SUPERPAGE_ORDER) - 1; is the more normal way of writing this.
>
> if ( !mfns || !pfns )
> {
> @@ -152,14 +179,26 @@ int populate_pfns(struct xc_sr_context *ctx, unsigned int count,
>
> for ( i = 0; i < count; ++i )
> {
> + xen_pfn_t pfn = original_pfns[i];
> +
> if ( (!types || page_type_to_populate(types[i])) &&
> - !pfn_is_populated(ctx, original_pfns[i]) )
> + !pfn_is_populated(ctx, pfn) )
> {
> - rc = pfn_set_populated(ctx, original_pfns[i]);
> + rc = pfn_set_populated(ctx, pfn);
> if ( rc )
> goto err;
> - pfns[nr_pfns] = mfns[nr_pfns] = original_pfns[i];
> + pfns[nr_pfns] = mfns[nr_pfns] = pfn;
> ++nr_pfns;
/* For x86 HVM guests in the first pass, PAGE_DATA records contain
metadata about 4M aligned chunks of GFN space. Reconstruct 2M
superpages where possible. */
I'm happy to fix these all on commit, if you're happy?
~Andrew
> + if ( pfn != prev + 1 )
> + num_contiguous = 0;
> + num_contiguous++;
> + prev = pfn;
> + if ( num_contiguous > mask && (pfn & mask) == mask &&
> + populate_small_superpage(ctx, pfn - mask) )
> + {
> + nr_pfns -= mask + 1;
> + num_contiguous = 0;
> + }
> }
> }
>
On Fri, Sep 12, 2025 at 7:01 PM Andrew Cooper <andrew.cooper3@citrix.com> wrote:
>
> On 12/09/2025 10:57 am, Frediano Ziglio wrote:
> > Try to allocate larger order pages.
> > With some test memory program stressing TLB (many small random
> > memory accesses) you can get 15% performance improves.
Checker is suggesting "improvements" instead of "improves" here. Can
you also update this ?
> > On the first memory iteration the sender is currently sending
> > memory in 4mb aligned chunks which allows the receiver to
> > allocate most pages as 2mb superpages instead of single 4kb pages.
> > This works even for HVM where the first 2mb contains some holes.
> > This change does not handle 1gb superpages as this will require
> > change in the protocol to preallocate space.
> >
> > Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com>
> > Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
>
> Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
>
> Thanks, this is far easier to follow. A couple of minor things.
>
> > ---
> > Changes since v1:
> > - updated commit message and subject;
> > - change the implementation detecting possible 2mb pages inside
> > the packet sent allowing more 2mb superpages.
> >
> > Changes since v2:
> > - change implementation simplifying detecting and allocations
> > of 2mb pages.
> > ---
> > tools/libs/guest/xg_sr_restore.c | 45 +++++++++++++++++++++++++++++---
> > 1 file changed, 42 insertions(+), 3 deletions(-)
> >
> > diff --git a/tools/libs/guest/xg_sr_restore.c b/tools/libs/guest/xg_sr_restore.c
> > index 06231ca826..ea5a137612 100644
> > --- a/tools/libs/guest/xg_sr_restore.c
> > +++ b/tools/libs/guest/xg_sr_restore.c
> > @@ -129,6 +129,30 @@ static int pfn_set_populated(struct xc_sr_context *ctx, xen_pfn_t pfn)
> > return 0;
> > }
> >
> > +#if defined(__i386__) || defined(__x86_64__)
> > +/* Order of the smallest superpage */
> > +#define SMALL_SUPERPAGE_ORDER 9
> > +#else
> > +#error Define SMALL_SUPERPAGE_ORDER for this platform
> > +#endif
> > +
> > +static bool populate_small_superpage(struct xc_sr_context *ctx, xen_pfn_t pfn)
>
> I know the terminology is terrible (this work was what prompted some of
> my clean-up attempts in Xen).
>
> I think we want to s/pfn/gfn/ all across this function.
>
> > +{
> > + xen_pfn_t mfn = pfn;
> > +
> > + if ( xc_domain_populate_physmap_exact(
> > + ctx->xch, ctx->domid, 1, SMALL_SUPERPAGE_ORDER, 0, &mfn) )
>
> This needs a comment.
>
> /* XENMEM_populate_physmap has no coherent error semantics. Assume a
> failure here is ENOMEM, and fall back to allocating small pages. */
>
> (Yes, the physmap hypercalls are insane. The only error feedback is "I
> completed this many before something went wrong", and libxenctrl chooses
> EBUSY for want of anything better.)
>
> > + return false;
> > +
> > + if ( mfn == INVALID_MFN )
> > + return false;
> > +
> > + for ( size_t i = 0; i < (1 << SMALL_SUPERPAGE_ORDER); ++i )
> > + ctx->restore.ops.set_gfn(ctx, pfn + i, mfn + i);
> > +
> > + return true;
> > +}
> > +
> > /*
> > * Given a set of pfns, obtain memory from Xen to fill the physmap for the
> > * unpopulated subset. If types is NULL, no page type checking is performed
> > @@ -142,6 +166,9 @@ int populate_pfns(struct xc_sr_context *ctx, unsigned int count,
> > *pfns = malloc(count * sizeof(*pfns));
> > unsigned int i, nr_pfns = 0;
> > int rc = -1;
> > + xen_pfn_t prev = 0;
> > + unsigned num_contiguous = 0;
> > + xen_pfn_t mask = ~((~(xen_pfn_t)0) << SMALL_SUPERPAGE_ORDER);
>
> (1ULL << SMALL_SUPERPAGE_ORDER) - 1; is the more normal way of writing this.
>
> >
> > if ( !mfns || !pfns )
> > {
> > @@ -152,14 +179,26 @@ int populate_pfns(struct xc_sr_context *ctx, unsigned int count,
> >
> > for ( i = 0; i < count; ++i )
> > {
> > + xen_pfn_t pfn = original_pfns[i];
> > +
> > if ( (!types || page_type_to_populate(types[i])) &&
> > - !pfn_is_populated(ctx, original_pfns[i]) )
> > + !pfn_is_populated(ctx, pfn) )
> > {
> > - rc = pfn_set_populated(ctx, original_pfns[i]);
> > + rc = pfn_set_populated(ctx, pfn);
> > if ( rc )
> > goto err;
> > - pfns[nr_pfns] = mfns[nr_pfns] = original_pfns[i];
> > + pfns[nr_pfns] = mfns[nr_pfns] = pfn;
> > ++nr_pfns;
>
> /* For x86 HVM guests in the first pass, PAGE_DATA records contain
> metadata about 4M aligned chunks of GFN space. Reconstruct 2M
> superpages where possible. */
>
> I'm happy to fix these all on commit, if you're happy?
>
Yes, fine with all changes.
> ~Andrew
>
Frediano
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