Update after design review

Author: damyanp

proposals/infra/INF-NNNN-validator-backcompat-testing.md

@@ -80,25 +80,61 @@ The `-select-validator` option will be deprecated.  As this is not documented,
 it should be ok to just remove it entirely.
 
 A new option, `-dxil-dll-path`, can be used to specify the full path to dxil.dll
-to use. DXC will also be modified to check for a new environment variable,
-`DXC_DXIL_DLL_PATH`, that also specifies the full path to the dll.  Example
-path: `c:\dxc\build\old-releases\1.7.123\bin\x64\dxil.dll`.
+to use. Example path: `c:\dxc\build\old-releases\1.7.123\bin\x64\dxil.dll`.
 
-The command-line option takes precedance over the environment variable. Setting
-the path to an empty string forces the internal validator.
+Setting the path to an empty string forces the internal validator to be used.
 
-If a dll path is specified and isn't found then DXC will fail with an error.
+If a dll path is specified and isn't found then the compiler will fail with an
+error.
+
+When invoked via dxc.exe (ie not through the compiler API), DXC will check for a
+new environment variable, `DXC_DXIL_DLL_PATH`, that  specifies the full path to
+the dll. The command-line option takes precedance over the environment variable.
+
+When an external validator is used a warning diagnostic is emitted. Since the
+only expected users of this option is the backcompat tests having this always
+appear in the test logs should be helpful when diagnosing issues.  
 
-Note that the environment variable is used for both API as well as command-line
-invocation of the compiler.
 
 ### Invoking the tests
 
 The tests are invoked via `add_custom_command`, using `cmake -E env` to set the
 `DXC_DXIL_DLL_PATH` variable.
 
+### New Tests
+
+As well as testing old external validators, we also need to run the same set of
+tests against current external validator.
+
+This means we need to test:
+
+* internal validator
+* latest external validator
+* previouslly released external validators
+
+## Alternatives Considered
+
+### Requirement for full path
+
+We considered allowing relative paths to specify the dll. However, best practice
+for loading DLLs is to use absolute paths and since in the expected scenarios
+for this functionality we know exactly which DLL we want to use the decision was
+made to require an absolute path to the DLL.
+
+
+### Diagnostic when using External Validator
+
+We considered that we should only show the diagnostic when the compiler is
+invoked via dxc.exe and not when it is used via the API, on the basis that
+emitting a warning when the API is doing something that the user explicitly
+asked it to do is unusual. 
 
+Another consideration was to only output the path to the validator in the case
+that validation failed.
 
+Since this API is only intended to be used scenarios for testing the compiler
+itself, the decision was made to always emit it in order to help with debugging
+via logs.