Loongarch: KVM: Add KVM hypercalls documentation for LoongArch

Add documentation topic for using pv_virt when running as a guest
on KVM hypervisor.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Xianglai Li <lixianglai@loongson.cn>
Co-developed-by: Mingcong Bai <jeffbai@aosc.io>
Signed-off-by: Mingcong Bai <jeffbai@aosc.io>
Link: https://lore.kernel.org/all/5c338084b1bcccc1d57dce9ddb1e7081@aosc.io/
Signed-off-by: Dandan Zhang <zhangdandan@uniontech.com>
[jc: fixed htmldocs build error]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/4769C036576F8816+20240828045950.3484113-1-zhangdandan@uniontech.com

diff --git a/Documentation/virt/kvm/index.rst b/Documentation/virt/kvm/index.rst
index ad13ec55ddfe5110ab8922a5aafe1732951209de..9ca5a45c2140a9f3dfc1dc58b28a871f4c3b844f 100644 (file)
--- a/Documentation/virt/kvm/index.rst
+++ b/Documentation/virt/kvm/index.rst
@@ -14,6 +14,7 @@ KVM
    s390/index
    ppc-pv
    x86/index
+   loongarch/index
 
    locking
    vcpu-requests

diff --git a/Documentation/virt/kvm/loongarch/hypercalls.rst b/Documentation/virt/kvm/loongarch/hypercalls.rst
new file mode 100644 (file)
index 0000000..2d6b940
--- /dev/null
+++ b/Documentation/virt/kvm/loongarch/hypercalls.rst
@@ -0,0 +1,89 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===================================
+The LoongArch paravirtual interface
+===================================
+
+KVM hypercalls use the HVCL instruction with code 0x100 and the hypercall
+number is put in a0. Up to five arguments may be placed in registers a1 - a5.
+The return value is placed in v0 (an alias of a0).
+
+Source code for this interface can be found in arch/loongarch/kvm*.
+
+Querying for existence
+======================
+
+To determine if the host is running on KVM, we can utilize the cpucfg()
+function at index CPUCFG_KVM_BASE (0x40000000).
+
+The CPUCFG_KVM_BASE range, spanning from 0x40000000 to 0x400000FF, The
+CPUCFG_KVM_BASE range between 0x40000000 - 0x400000FF is marked as reserved.
+Consequently, all current and future processors will not implement any
+feature within this range.
+
+On a KVM-virtualized Linux system, a read operation on cpucfg() at index
+CPUCFG_KVM_BASE (0x40000000) returns the magic string 'KVM\0'.
+
+Once you have determined that your host is running on a paravirtualization-
+capable KVM, you may now use hypercalls as described below.
+
+KVM hypercall ABI
+=================
+
+The KVM hypercall ABI is simple, with one scratch register a0 (v0) and at most
+five generic registers (a1 - a5) used as input parameters. The FP (Floating-
+point) and vector registers are not utilized as input registers and must
+remain unmodified during a hypercall.
+
+Hypercall functions can be inlined as it only uses one scratch register.
+
+The parameters are as follows:
+
+       ========        =================       ================
+       Register        IN                      OUT
+       ========        =================       ================
+       a0              function number         Return  code
+       a1              1st     parameter       -
+       a2              2nd     parameter       -
+       a3              3rd     parameter       -
+       a4              4th     parameter       -
+       a5              5th     parameter       -
+       ========        =================       ================
+
+The return codes may be one of the following:
+
+       ====            =========================
+       Code            Meaning
+       ====            =========================
+       0               Success
+       -1              Hypercall not implemented
+       -2              Bad Hypercall parameter
+       ====            =========================
+
+KVM Hypercalls Documentation
+============================
+
+The template for each hypercall is as follows:
+
+1. Hypercall name
+2. Purpose
+
+1. KVM_HCALL_FUNC_IPI
+------------------------
+
+:Purpose: Send IPIs to multiple vCPUs.
+
+- a0: KVM_HCALL_FUNC_IPI
+- a1: Lower part of the bitmap for destination physical CPUIDs
+- a2: Higher part of the bitmap for destination physical CPUIDs
+- a3: The lowest physical CPUID in the bitmap
+
+The hypercall lets a guest send multiple IPIs (Inter-Process Interrupts) with
+at most 128 destinations per hypercall. The destinations are represented in a
+bitmap contained in the first two input registers (a1 and a2).
+
+Bit 0 of a1 corresponds to the physical CPUID in the third input register (a3)
+and bit 1 corresponds to the physical CPUID in a3+1, and so on.
+
+PV IPI on LoongArch includes both PV IPI multicast sending and PV IPI receiving,
+and SWI is used for PV IPI inject since there is no VM-exits accessing SWI registers.

diff --git a/Documentation/virt/kvm/loongarch/index.rst b/Documentation/virt/kvm/loongarch/index.rst
new file mode 100644 (file)
index 0000000..83387b4
--- /dev/null
+++ b/Documentation/virt/kvm/loongarch/index.rst
@@ -0,0 +1,10 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=========================
+KVM for LoongArch systems
+=========================
+
+.. toctree::
+   :maxdepth: 2
+
+   hypercalls.rst

diff --git a/MAINTAINERS b/MAINTAINERS
index 9d30466b14a79bd8046f2047399f2320a193a952..a556c90a8b5720c66573000f39f26c167ae860c3 100644 (file)
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -12299,6 +12299,7 @@ L:      kvm@vger.kernel.org
 L:     loongarch@lists.linux.dev
 S:     Maintained
 T:     git git://git.kernel.org/pub/scm/virt/kvm/kvm.git
+F:     Documentation/virt/kvm/loongarch/
 F:     arch/loongarch/include/asm/kvm*
 F:     arch/loongarch/include/uapi/asm/kvm*
 F:     arch/loongarch/kvm/