On 01/09/2017 12:37 AM, Adrian.Danis@data61.csiro.au wrote:
Hi Corey,
The original intention of the 'aggressive' check was to somewhat prevent your case where the user has set the size higher than required and may be inadvertently be wasting memory by having the TCBBits end up larger than needed. I agree that it can be a little inconvenient, so will change it allow sizes that are larger, but emit a warning.
Sounds great, thanks!
Comparing against ebx instead of ecx is quite deliberate as this way we only measure the size required for the features that the user actually wanted to use as defined in CONFIG_XSAVE_FEATURE_SET. This is why we do this check just after performing 'write_xcr0' with said desired features.
Hm, this isn't consistent with my observations of the actual behavior, although I'm not seeing why at this point.
On my Ivy Bridge Xeon CPU (supports AVX) with CONFIG_XSAVE_FEATURE_SET=3, using Qemu/KVM with -cpu host and the linked kernel config (and a small patch to always print out ebx/ecx), I get 576 for ebx and 832 for ecx.
Kernel config: http://ix.io/1PCY
My only patch from 4.0.0: http://ix.io/1PCZ
Kernel log: http://ix.io/1PD3
Qemu invocation:
qemu-system-x86_64 -cpu host,+avx -enable-kvm -nographic -kernel ~/proj/robigalia/sel4/stage/kernel-x86_64-pc99 -initrd ~/proj/robigalia/hello-world/target/x86_64-sel4-robigalia/release/hello-world
Maybe there's another issue at play here?