On Fri, Jun 24, 2022 at 10:37:10AM -0600, Peter Gonda wrote: > On Tue, Jun 14, 2022 at 6:03 AM Kirill A. Shutemov > <kirill.shutemov@xxxxxxxxxxxxxxx> wrote: > > > > UEFI Specification version 2.9 introduces the concept of memory > > acceptance: some Virtual Machine platforms, such as Intel TDX or AMD > > SEV-SNP, requiring memory to be accepted before it can be used by the > > guest. Accepting happens via a protocol specific for the Virtual > > Machine platform. > > > > Accepting memory is costly and it makes VMM allocate memory for the > > accepted guest physical address range. It's better to postpone memory > > acceptance until memory is needed. It lowers boot time and reduces > > memory overhead. > > > > The kernel needs to know what memory has been accepted. Firmware > > communicates this information via memory map: a new memory type -- > > EFI_UNACCEPTED_MEMORY -- indicates such memory. > > > > Range-based tracking works fine for firmware, but it gets bulky for > > the kernel: e820 has to be modified on every page acceptance. It leads > > to table fragmentation, but there's a limited number of entries in the > > e820 table > > > > Another option is to mark such memory as usable in e820 and track if the > > range has been accepted in a bitmap. One bit in the bitmap represents > > 2MiB in the address space: one 4k page is enough to track 64GiB or > > physical address space. > > > > In the worst-case scenario -- a huge hole in the middle of the > > address space -- It needs 256MiB to handle 4PiB of the address > > space. > > > > Any unaccepted memory that is not aligned to 2M gets accepted upfront. > > > > The approach lowers boot time substantially. Boot to shell is ~2.5x > > faster for 4G TDX VM and ~4x faster for 64G. > > > > TDX-specific code isolated from the core of unaccepted memory support. It > > supposed to help to plug-in different implementation of unaccepted memory > > such as SEV-SNP. > > > > The tree can be found here: > > > > https://github.com/intel/tdx.git guest-unaccepted-memory > > Hi Kirill, > > I have a couple questions about this feature mainly about how cloud > customers can use this, I assume since this is a confidential compute > feature a large number of the users of these patches will be cloud > customers using TDX and SNP. One issue I see with these patches is how > do we as a cloud provider know whether a customer's linux image > supports this feature, if the image doesn't have these patches UEFI > needs to fully validate the memory, if the image does we can use this > new protocol. In GCE we supply our VMs with a version of the EDK2 FW > and the customer doesn't input into which UEFI we run, as far as I can > tell from the Azure SNP VM documentation it seems very similar. We > need to somehow tell our UEFI in the VM what to do based on the image. > The current way I can see to solve this issue would be to have our > customers give us metadata about their VM's image but this seems kinda > burdensome on our customers (I assume we'll have more features which > both UEFI and kernel need to both support inorder to be turned on like > this one) and error-prone, if a customer incorrectly labels their > image it may fail to boot.. Has there been any discussion about how to > solve this? My naive thoughts were what if UEFI and Kernel had some > sort of feature negotiation. Maybe that could happen via an extension > to exit boot services or a UEFI runtime driver, I'm not sure what's > best here just some ideas. Just as an idea, we can put info into UTS_VERSION which can be read from the built bzImage. We have info on SMP and preeption there already. Patch below does this: $ file arch/x86/boot/bzImage arch/x86/boot/bzImage: Linux kernel x86 boot executable bzImage, version 5.19.0-rc3-00016-g2f6aa48e28d9-dirty (kas@box) #2300 SMP PREEMPT_DYNAMIC UNACCEPTED_MEMORY Mon Jun 27 14:23:04 , RO-rootFS, swap_dev 0XC, Normal VGA Note UNACCEPTED_MEMORY in the output. Probably we want to have there info on which flavour of unaccepted memory is supported (TDX/SNP/whatever). It is a bit more tricky. Any opinion? diff --git a/init/Makefile b/init/Makefile index d82623d7fc8e..6688ea43e6bf 100644 --- a/init/Makefile +++ b/init/Makefile @@ -32,7 +32,7 @@ quiet_cmd_compile.h = CHK $@ $(CONFIG_SHELL) $(srctree)/scripts/mkcompile_h $@ \ "$(UTS_MACHINE)" "$(CONFIG_SMP)" "$(CONFIG_PREEMPT_BUILD)" \ "$(CONFIG_PREEMPT_DYNAMIC)" "$(CONFIG_PREEMPT_RT)" \ - "$(CONFIG_CC_VERSION_TEXT)" "$(LD)" + "$(CONFIG_UNACCEPTED_MEMORY)" "$(CONFIG_CC_VERSION_TEXT)" "$(LD)" include/generated/compile.h: FORCE $(call cmd,compile.h) diff --git a/scripts/mkcompile_h b/scripts/mkcompile_h index ca40a5258c87..efacfecad699 100755 --- a/scripts/mkcompile_h +++ b/scripts/mkcompile_h @@ -7,8 +7,9 @@ SMP=$3 PREEMPT=$4 PREEMPT_DYNAMIC=$5 PREEMPT_RT=$6 -CC_VERSION="$7" -LD=$8 +UNACCEPTED_MEMORY=$7 +CC_VERSION="$8" +LD=$9 # Do not expand names set -f @@ -51,6 +52,10 @@ elif [ -n "$PREEMPT" ] ; then CONFIG_FLAGS="$CONFIG_FLAGS PREEMPT" fi +if [ -n "$UNACCEPTED_MEMORY" ] ; then + CONFIG_FLAGS="$CONFIG_FLAGS UNACCEPTED_MEMORY" +fi + # Truncate to maximum length UTS_LEN=64 UTS_VERSION="$(echo $UTS_VERSION $CONFIG_FLAGS $TIMESTAMP | cut -b -$UTS_LEN)" -- Kirill A. Shutemov
