Re: [PATCH v6 5/7] KVM: x86: Introduce untag_addr() in kvm_x86_ops

[Chao Gao <chao.gao@intel.com>]

On Sun, Mar 19, 2023 at 04:49:25PM +0800, Binbin Wu wrote:
>Introduce a new interface untag_addr() to kvm_x86_ops to untag the metadata
>from linear address. Implement LAM version in VMX and dummy version in SVM.
>
>When enabled feature like Intel Linear Address Masking or AMD Upper
>Address Ignore, linear address may be tagged with metadata. Linear
>address should be checked for modified canonicality and untagged in
>instrution emulations or vmexit handlings if LAM or UAI is applicable.
>
>Introduce untag_addr() to kvm_x86_ops to hide the code related to vendor
>specific details.
>- For VMX, LAM version is implemented.
>  LAM has a modified canonical check when applicable:
>  * LAM_S48                : [ 1 ][ metadata ][ 1 ]
>                               63               47
>  * LAM_U48                : [ 0 ][ metadata ][ 0 ]
>                               63               47
>  * LAM_S57                : [ 1 ][ metadata ][ 1 ]
>                               63               56
>  * LAM_U57 + 5-lvl paging : [ 0 ][ metadata ][ 0 ]
>                               63               56
>  * LAM_U57 + 4-lvl paging : [ 0 ][ metadata ][ 0...0 ]
>                               63               56..47
>  If LAM is applicable to certain address, untag the metadata bits and
>  replace them with the value of bit 47 (LAM48) or bit 56 (LAM57). Later
>  the untagged address will do legacy canonical check. So that LAM canonical
>  check and mask can be covered by "untag + legacy canonical check".
>
>  For cases LAM is not applicable, 'flags' is passed to the interface
>  to skip untag.
>
>- For SVM, add a dummy version to do nothing, but return the original
>  address.
>
>Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
>---
> arch/x86/include/asm/kvm-x86-ops.h |  1 +
> arch/x86/include/asm/kvm_host.h    |  5 +++
> arch/x86/kvm/svm/svm.c             |  7 ++++
> arch/x86/kvm/vmx/vmx.c             | 60 ++++++++++++++++++++++++++++++
> arch/x86/kvm/vmx/vmx.h             |  2 +
> 5 files changed, 75 insertions(+)
>
>diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
>index 8dc345cc6318..7d63d1b942ac 100644
>--- a/arch/x86/include/asm/kvm-x86-ops.h
>+++ b/arch/x86/include/asm/kvm-x86-ops.h
>@@ -52,6 +52,7 @@ KVM_X86_OP(cache_reg)
> KVM_X86_OP(get_rflags)
> KVM_X86_OP(set_rflags)
> KVM_X86_OP(get_if_flag)
>+KVM_X86_OP(untag_addr)

Suppose AMD doesn't/won't use CR4.LAM_SUP and CR3.LAM_U48/U57 for other
purposes, it is fine to use a common x86 function to perform LAM masking
for pointers. It won't do anything harmful on AMD parts because those
enabling bits shouldn't be set and then no bits will be masked out by
the common x86 function.

Probably we can defer the introduction of the hook to when the
assumption becomes wrong.