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d="scan'208";a="123474049" From: Andrew Cooper To: Xen-devel CC: Michal Orzel , Andrew Cooper , George Dunlap , "Jan Beulich" , Stefano Stabellini , "Wei Liu" , Julien Grall , Henry Wang Subject: [PATCH for-4.18 v2 2/2] x86: Clarify that only 5 hypercall parameters are supported Date: Fri, 6 Oct 2023 19:54:02 +0100 Message-ID: <20231006185402.1098400-3-andrew.cooper3@citrix.com> X-Mailer: git-send-email 2.30.2 In-Reply-To: <20231006185402.1098400-1-andrew.cooper3@citrix.com> References: <20231006185402.1098400-1-andrew.cooper3@citrix.com> MIME-Version: 1.0 From: Michal Orzel The x86 hypercall ABI really used to have 6-argument hypercalls. V4V, the downstream predecessor to Argo did take 6th args. However, the 6th arg being %ebp in the 32bit ABI makes it unusable in practice, because that's the frame pointer in builds with frame pointers enabled. Therefore Argo was altered to being a 5-arg hypercall when it was upstreamed. c/s 2f531c122e95 ("x86: limit number of hypercall parameters to 5") removed the ability for hypercalls to take 6 arguments. Update the documentation to match reality. Signed-off-by: Michal Orzel Signed-off-by: Andrew Cooper Reviewed-by: Jason Andryuk --- CC: George Dunlap CC: Jan Beulich CC: Stefano Stabellini CC: Wei Liu CC: Julien Grall CC: Michal Orzel CC: Henry Wang v2: * Extend with the historical context of how 6-arg hypercalls have come and gone. --- docs/guest-guide/x86/hypercall-abi.rst | 15 +++++++++++---- xen/include/public/arch-x86/xen-x86_32.h | 2 +- xen/include/public/arch-x86/xen-x86_64.h | 2 +- 3 files changed, 13 insertions(+), 6 deletions(-) diff --git a/docs/guest-guide/x86/hypercall-abi.rst b/docs/guest-guide/x86/hypercall-abi.rst index 42a820386b68..c7a11a76712f 100644 --- a/docs/guest-guide/x86/hypercall-abi.rst +++ b/docs/guest-guide/x86/hypercall-abi.rst @@ -4,7 +4,7 @@ Hypercall ABI ============= Hypercalls are system calls to Xen. Two modes of guest operation are -supported, and up to 6 individual parameters are supported. +supported, and up to 5 individual parameters are supported. Hypercalls may only be issued by kernel-level software [#kern]_. @@ -18,17 +18,17 @@ The registers used for hypercalls depends on the operating mode of the guest. * - ABI - Hypercall Index - - Parameters (1 - 6) + - Parameters (1 - 5) [#params]_ - Result * - 64bit - RAX - - RDI RSI RDX R10 R8 R9 + - RDI RSI RDX R10 R8 - RAX * - 32bit - EAX - - EBX ECX EDX ESI EDI EBP + - EBX ECX EDX ESI EDI - EAX 32 and 64bit PV guests have an ABI fixed by their guest type. The ABI for an @@ -119,6 +119,13 @@ means. .. [#kern] For HVM guests, ``HVMOP_guest_request_vm_event`` may be configured to be usable from userspace, but this behaviour is not default. +.. [#params] Xen's ABI used to declare support for 6 hypercall arguments, + using ``r9`` and ``ebp``. However, such an ABI clobbers the frame pointer + in the 32bit code and does interact nicely with guest-side debugging. The + predecessor to ``HYPERCALL_argo_op`` was a 6-argument hypercall, but the + ABI was intentionally altered when Argo was upstreamed (Xen 4.13) to be the + 5-argument hypercall it now is. + .. [#mode] While it is possible to use compatibility mode segments in a 64bit kernel, hypercalls issues from such a mode will be interpreted with the 32bit ABI. Such a setup is not expected in production scenarios. diff --git a/xen/include/public/arch-x86/xen-x86_32.h b/xen/include/public/arch-x86/xen-x86_32.h index 139438e83534..9e3bf06b121e 100644 --- a/xen/include/public/arch-x86/xen-x86_32.h +++ b/xen/include/public/arch-x86/xen-x86_32.h @@ -12,7 +12,7 @@ /* * Hypercall interface: - * Input: %ebx, %ecx, %edx, %esi, %edi, %ebp (arguments 1-6) + * Input: %ebx, %ecx, %edx, %esi, %edi (arguments 1-5) * Output: %eax * Access is via hypercall page (set up by guest loader or via a Xen MSR): * call hypercall_page + hypercall-number * 32 diff --git a/xen/include/public/arch-x86/xen-x86_64.h b/xen/include/public/arch-x86/xen-x86_64.h index 5d9035ed2230..43f6e3d22001 100644 --- a/xen/include/public/arch-x86/xen-x86_64.h +++ b/xen/include/public/arch-x86/xen-x86_64.h @@ -12,7 +12,7 @@ /* * Hypercall interface: - * Input: %rdi, %rsi, %rdx, %r10, %r8, %r9 (arguments 1-6) + * Input: %rdi, %rsi, %rdx, %r10, %r8 (arguments 1-5) * Output: %rax * Access is via hypercall page (set up by guest loader or via a Xen MSR): * call hypercall_page + hypercall-number * 32