Conjugate Gradient technique

[Barai, Pallab]

Hello,

I am using PETSc to solve a set of linear equations "Sx=b".

Here "b" is known and "x" is the trial solution.

The complete (assembled) form of S is not known. That is why I am not able to use something like "KSPSolve". 

But given a trial solution "x", I can calculate "S*x" using a "MatVec" routine. 

Is it possible to use the Conjugate Gradient (CG) technique to find a solution in this case? In place of "S", I can give "S*x" as the input. 

It will be great if someone can show me some direction. If this thing has already been discussed before, the link to that thread will be sufficient.

Thanking you.

Pallab Barai