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Re: [PATCH v2 15/18] target/riscv: make riscv_isa_string_ext() KVM compa

From: Daniel Henrique Barboza
Subject: Re: [PATCH v2 15/18] target/riscv: make riscv_isa_string_ext() KVM compatible
Date: Tue, 20 Jun 2023 19:05:18 -0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.12.0

On 6/19/23 06:54, Andrew Jones wrote:
On Tue, Jun 13, 2023 at 05:58:54PM -0300, Daniel Henrique Barboza wrote:
The isa_edata_arr[] is used by riscv_isa_string_ext() to create the
riscv,isa DT attribute. isa_edata_arr[] is kept in sync with the TCG
property vector riscv_cpu_extensions[], i.e. all TCG properties from
this vector that has a riscv,isa representation are included in

KVM doesn't implement all TCG properties, but allow them to be created
anyway to not force an API change between TCG and KVM guests. Some of
these TCG-only extensions are defaulted to 'true', and users are also
allowed to enable them. KVM doesn't care, but riscv_isa_string_ext()
does. The result is that these TCG-only enabled extensions will appear
in the riscv,isa DT string under KVM.

To avoid code repetition and re-use riscv_isa_string_ext() for KVM
guests we'll make a couple of tweaks:

- set env->priv_ver to 'LATEST' for the KVM 'host' CPU. This is needed
   because riscv_isa_string_ext() makes a priv check for each extension
   before including them in the ISA string. KVM doesn't care about
   env->priv_ver, since it's part of the TCG-only CPU validation, so this
   change is benign for KVM;

- add a new 'kvm_available' flag in isa_ext_data struct. This flag is
   set via a new ISA_EXT_DATA_ENTRY_KVM macro to report that, for a given
   extension, KVM also supports it. riscv_isa_string_ext() then can check
   if a given extension is known by KVM and skip it if it's not.

This will allow us to re-use riscv_isa_string_ext() for KVM guests.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
  target/riscv/cpu.c | 28 ++++++++++++++++++++--------
  1 file changed, 20 insertions(+), 8 deletions(-)

diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
index a4f3ed0c17..a773c09645 100644
--- a/target/riscv/cpu.c
+++ b/target/riscv/cpu.c
@@ -44,11 +44,15 @@ struct isa_ext_data {
      const char *name;
      int min_version;
      int ext_enable_offset;
+    bool kvm_available;
#define ISA_EXT_DATA_ENTRY(_name, _min_ver, _prop) \
      {#_name, _min_ver, offsetof(struct RISCVCPUConfig, _prop)}
+#define ISA_EXT_DATA_ENTRY_KVM(_name, _min_ver, _prop) \
+    {#_name, _min_ver, offsetof(struct RISCVCPUConfig, _prop), true}
   * Here are the ordering rules of extension naming defined by RISC-V
   * specification :
@@ -68,14 +72,17 @@ struct isa_ext_data {
   * Single letter extensions are checked in riscv_cpu_validate_misa_priv()
   * instead.
+ *
+ * ISA_EXT_DATA_ENTRY_KVM() is used to indicate that the extension is
+ * also known by the KVM driver. If unsure, use ISA_EXT_DATA_ENTRY().
  static const struct isa_ext_data isa_edata_arr[] = {
-    ISA_EXT_DATA_ENTRY(zicbom, PRIV_VERSION_1_12_0, ext_icbom),
-    ISA_EXT_DATA_ENTRY(zicboz, PRIV_VERSION_1_12_0, ext_icboz),
+    ISA_EXT_DATA_ENTRY_KVM(zicbom, PRIV_VERSION_1_12_0, ext_icbom),
+    ISA_EXT_DATA_ENTRY_KVM(zicboz, PRIV_VERSION_1_12_0, ext_icboz),
      ISA_EXT_DATA_ENTRY(zicond, PRIV_VERSION_1_12_0, ext_zicond),
      ISA_EXT_DATA_ENTRY(zicsr, PRIV_VERSION_1_10_0, ext_icsr),
      ISA_EXT_DATA_ENTRY(zifencei, PRIV_VERSION_1_10_0, ext_ifencei),
-    ISA_EXT_DATA_ENTRY(zihintpause, PRIV_VERSION_1_10_0, ext_zihintpause),
+    ISA_EXT_DATA_ENTRY_KVM(zihintpause, PRIV_VERSION_1_10_0, ext_zihintpause),
      ISA_EXT_DATA_ENTRY(zawrs, PRIV_VERSION_1_12_0, ext_zawrs),
      ISA_EXT_DATA_ENTRY(zfh, PRIV_VERSION_1_11_0, ext_zfh),
      ISA_EXT_DATA_ENTRY(zfhmin, PRIV_VERSION_1_11_0, ext_zfhmin),
@@ -89,7 +96,7 @@ static const struct isa_ext_data isa_edata_arr[] = {
      ISA_EXT_DATA_ENTRY(zcmp, PRIV_VERSION_1_12_0, ext_zcmp),
      ISA_EXT_DATA_ENTRY(zcmt, PRIV_VERSION_1_12_0, ext_zcmt),
      ISA_EXT_DATA_ENTRY(zba, PRIV_VERSION_1_12_0, ext_zba),
-    ISA_EXT_DATA_ENTRY(zbb, PRIV_VERSION_1_12_0, ext_zbb),
+    ISA_EXT_DATA_ENTRY_KVM(zbb, PRIV_VERSION_1_12_0, ext_zbb),
      ISA_EXT_DATA_ENTRY(zbc, PRIV_VERSION_1_12_0, ext_zbc),
      ISA_EXT_DATA_ENTRY(zbkb, PRIV_VERSION_1_12_0, ext_zbkb),
      ISA_EXT_DATA_ENTRY(zbkc, PRIV_VERSION_1_12_0, ext_zbkc),
@@ -114,13 +121,13 @@ static const struct isa_ext_data isa_edata_arr[] = {
      ISA_EXT_DATA_ENTRY(zhinxmin, PRIV_VERSION_1_12_0, ext_zhinxmin),
      ISA_EXT_DATA_ENTRY(smaia, PRIV_VERSION_1_12_0, ext_smaia),
      ISA_EXT_DATA_ENTRY(smstateen, PRIV_VERSION_1_12_0, ext_smstateen),
-    ISA_EXT_DATA_ENTRY(ssaia, PRIV_VERSION_1_12_0, ext_ssaia),
+    ISA_EXT_DATA_ENTRY_KVM(ssaia, PRIV_VERSION_1_12_0, ext_ssaia),
      ISA_EXT_DATA_ENTRY(sscofpmf, PRIV_VERSION_1_12_0, ext_sscofpmf),
-    ISA_EXT_DATA_ENTRY(sstc, PRIV_VERSION_1_12_0, ext_sstc),
+    ISA_EXT_DATA_ENTRY_KVM(sstc, PRIV_VERSION_1_12_0, ext_sstc),
      ISA_EXT_DATA_ENTRY(svadu, PRIV_VERSION_1_12_0, ext_svadu),
-    ISA_EXT_DATA_ENTRY(svinval, PRIV_VERSION_1_12_0, ext_svinval),
+    ISA_EXT_DATA_ENTRY_KVM(svinval, PRIV_VERSION_1_12_0, ext_svinval),
      ISA_EXT_DATA_ENTRY(svnapot, PRIV_VERSION_1_12_0, ext_svnapot),
-    ISA_EXT_DATA_ENTRY(svpbmt, PRIV_VERSION_1_12_0, ext_svpbmt),
+    ISA_EXT_DATA_ENTRY_KVM(svpbmt, PRIV_VERSION_1_12_0, ext_svpbmt),

This approach looks a bit difficult to maintain (it's hard to even see the
_KVM macro uses). The approach will be even more difficult if we add more
accelerators. I feel like we need an extension class where objects of that
class can be passed to functions like riscv_isa_string_ext(). And then we
also need tcg-extension and kvm-extension classes which inherit from
the extension class. We can then keep the lists of extensions separate, as
you originally proposed, as each list will be of its own type.

So, after coding around a little, I didn't manage to create a KVM list in
kvm.c file because of how ARRAY_SIZE() (a macro that riscv_isa_string_ext())
calculates the array size. If the list is exported from another file the macro
fails to calculate the size of the array. Similar issues also happens when 
to use kvm_multi_ext_cfgs[] in this function.

This means that both tcg and kvm arrays ended up in cpu.c. The tcg array is
left untouched (isa_edata_arr). The KVM array uses the same type TCG already
uses (isa_ext_edata) because it's good enough for KVM as well. I removed the
'kvm_available' flag since we're going to manually edit the array.

riscv_isa_string_ext() is then changed to switch between the tcg/kvm array
as required. The rest of the logic of the function stood the same. I'll also
add a pre-patch prior to all these changes to simplify riscv_isa_string_ext()
a bit when running under TCG.

In the end, as the code stands today, the only way to use this function with
the array we already have in KVM (kvm_multi_ext_cfgs[]) requires way more
changes than I believe it is worth doing ATM. I'd say that the extra kvm array
in cpu.c is an okay price to pay to keep the code in the simple side.




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