Messages in this thread | | | Date | Tue, 20 Sep 2022 18:15:07 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCH v4 1/4] x86/sev: Put PSC struct on the stack in prep for unaccepted memory support |
| |
On Thu, Aug 25, 2022 at 09:23:14AM -0500, Tom Lendacky wrote: > In advance of providing support for unaccepted memory, switch from using > kmalloc() for allocating the Page State Change (PSC) structure to using a > local variable that lives on the stack. This is needed to avoid a possible > recursive call into set_pages_state() if the kmalloc() call requires > (more) memory to be accepted, which would result in a hang. > > The current size of the PSC struct is 2,032 bytes. To make the struct more > stack friendly, reduce the number of PSC entries from 253 down to 64, > resulting in a size of 520 bytes. This is a nice compromise on struct size > and total PSC requests while still allowing parallel PSC operations across > vCPUs. > > If the reduction in PSC entries results in any kind of performance issue > (that is not seen at the moment), use of a larger static PSC struct, with > fallback to the smaller stack version, can be investigated.
"For more background info on this decision see the subthread in the Link tag below."
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/lkml/658c455c40e8950cb046dd885dd19dc1c52d060a.1659103274.git.thomas.lendacky@amd.com
> --- > arch/x86/include/asm/sev-common.h | 9 +++++++-- > arch/x86/kernel/sev.c | 10 ++-------- > 2 files changed, 9 insertions(+), 10 deletions(-) > > diff --git a/arch/x86/include/asm/sev-common.h b/arch/x86/include/asm/sev-common.h > index b8357d6ecd47..6c3d61c5f6a3 100644 > --- a/arch/x86/include/asm/sev-common.h > +++ b/arch/x86/include/asm/sev-common.h > @@ -106,8 +106,13 @@ enum psc_op { > #define GHCB_HV_FT_SNP BIT_ULL(0) > #define GHCB_HV_FT_SNP_AP_CREATION BIT_ULL(1) > > -/* SNP Page State Change NAE event */ > -#define VMGEXIT_PSC_MAX_ENTRY 253 > +/* > + * SNP Page State Change NAE event > + * The VMGEXIT_PSC_MAX_ENTRY determines the size of the PSC structure, > + * which is a local variable (stack usage) in set_pages_state(). Do not
... which is a local stack variable...
> + * increase this value without evaluating the impact to stack usage. > + */ ...
-- Regards/Gruss, Boris.
https://people.kernel.org/tglx/notes-about-netiquette
| |