diff options
Diffstat (limited to 'gitlab/issues_text/target_i386/host_missing/accel_TCG/844')
| -rw-r--r-- | gitlab/issues_text/target_i386/host_missing/accel_TCG/844 | 44 |
1 files changed, 0 insertions, 44 deletions
diff --git a/gitlab/issues_text/target_i386/host_missing/accel_TCG/844 b/gitlab/issues_text/target_i386/host_missing/accel_TCG/844 deleted file mode 100644 index a1fb8dd2b..000000000 --- a/gitlab/issues_text/target_i386/host_missing/accel_TCG/844 +++ /dev/null @@ -1,44 +0,0 @@ -Close gap for x86_64-v3 ABI in TCG - CPU support for fma, f16c, avx, avx2 features required -Description of problem: -There are 3 additional ABIs defined by a collaboration of vendors for the `x86_64` architecture, over the original baseline: - -* https://gitlab.com/x86-psABIs/x86-64-ABI/-/blob/master/x86-64-ABI/low-level-sys-info.tex - -This is no problem for KVM assuming suitable host hardware, but TCG is currently unable to support more than the original baseline and the `x86_64-v2` step. - -For `x86_64-v3` there are some gaps in its emulation coverage. This can be seen by taking `Nehalem` which is a good fit for `x86_64-v2`, and requesting the extra v3 features: - -``` -$ qemu-system-x86_64 -accel tcg -cpu Nehalem,+avx,+avx2,+bmi1,+bmi2,+f16c,+fma,+abm,+movbe -qemu-system-x86_64: warning: TCG doesn't support requested feature: CPUID.01H:ECX.fma [bit 12] -qemu-system-x86_64: warning: TCG doesn't support requested feature: CPUID.01H:ECX.avx [bit 28] -qemu-system-x86_64: warning: TCG doesn't support requested feature: CPUID.01H:ECX.f16c [bit 29] -qemu-system-x86_64: warning: TCG doesn't support requested feature: CPUID.07H:EBX.avx2 [bit 5] -``` - -IOW, the strict bare minimum TCG needs in order to satisfy `x86_64-v3` is `fma`, `f16c`, `avx` and `avx2` support - -If we want to fully support a named CPU model satisfying v3, then `Haswell` is the closest and that has a few additional gaps - -``` -$ qemu-system-x86_64 -accel tcg -cpu Haswell-noTSX -qemu-system-x86_64: warning: TCG doesn't support requested feature: CPUID.01H:ECX.fma [bit 12] -qemu-system-x86_64: warning: TCG doesn't support requested feature: CPUID.01H:ECX.pcid [bit 17] -qemu-system-x86_64: warning: TCG doesn't support requested feature: CPUID.01H:ECX.x2apic [bit 21] -qemu-system-x86_64: warning: TCG doesn't support requested feature: CPUID.01H:ECX.tsc-deadline [bit 24] -qemu-system-x86_64: warning: TCG doesn't support requested feature: CPUID.01H:ECX.avx [bit 28] -qemu-system-x86_64: warning: TCG doesn't support requested feature: CPUID.01H:ECX.f16c [bit 29] -qemu-system-x86_64: warning: TCG doesn't support requested feature: CPUID.07H:EBX.avx2 [bit 5] -qemu-system-x86_64: warning: TCG doesn't support requested feature: CPUID.07H:EBX.invpcid [bit 10] - -``` - -Those additional gaps wouldn't impact ability to execute binaries build for the `x86_64-v3` ABI though, so not as important. - -The reason `x86_64-v3` compatibility in TCG matters is because sooner or later some Linux OS are going to set this as the baseline for their compiler toolchain. There is a proposal to set this in `Fedora ELN`, which is what feeds in to a possible future `RHEL-10`. - -I imagine adding these extra features would be non-negligible work in TCG / take some time to complete. - -Thus I file this bug for the purpose of suggesting these 4 specific missing features be considered a priority to address, compared to other missing CPU features in TCG that might be considered more of a 'nice to have'. - -eg looking further the `x86_64-v4` baseline brings in a requirement for `avx512f`, `avx512bw`, `avx512cd`, `avx512dq`, `avx512vl` which TCG also lacks, but I don't think they really need to be considered important at this point in time. |